Weird daydream, how would you DM this?

[Hollysword]

I had a weird daydream yesterday, it felt like an interesting D&D adventure...

I saw myself sitting in a field in a forest, with a drow girl. She's trying to move away from the usual evil drow, find peace. I gave her some flowers from the fields, it was fine, until a group of drow attacks. Beat me up and took the girl away. My friends found me and took me to a nearby elf town, who gladly helped patch me up and provide info for us to track down the drow and save my friend who got taken (because elves hate the drow, and would offer any assistance to fight them, but didn't mention this friend is also a drow). We find the drow group, and found the drow girl is now a drider. We killed the drow, but the girl lost her mind being turned into a drider, and attacked us. My group subdued her, and when I gave her the same flowers from the field, she remembers and is back to her own mind. We went back to town together.

And that's when my daydream ended.

Now I'm thinking, how would the elves react that my missing friend is now a drider? How would you DM this awkward moment? Time for a critical diplomacy check?

Note, I've made it clear that she is my friend and not to be killed, and I will defend her if I have to. If she loses her mind again, I will bring her back myself. And even though I'm Good, I will kill if I need to, to defend the friend drider.

[El'the Ellie]

Needless to say, stuff below is just my opinion. But that's what you're asking for, so here we go!

To say the society would be unaccepting is an understatement. Consider that to an elven society, drow are the boogeymen. They are the creatures under your bed. Even your prompt shows they can and do kidnap people and smuggle them away in the night to do horrible things to them.
You can argue that some of the more worldly members of the town (elders, scholars, warriors, and so on) know enough about drow to see past the ghost stories, but that just makes their hate stronger: it's rooted in real loss of loved ones, real harass and torture at the hands of these people. 'Elves hate drow', as you said, but its not a groundless disdain. Drow have commit countless unspeakable acts against their kind. Elves have done the same in defense (legitimate or not). And so the cycle goes, with each generation preying on the other more and more, giving fresh new tragedies and slain loved ones and hate. Those that attempt to break the cycle are one in a million. Just that: one per million. For every story of a drow that renounced the ways of their culture, there are dozens stories of drow that tricked poor idiots into believing them, and countless stories of villains that ruined lives because it's fun. Because they worship a goddess that delights in the torture of elves.

And that's regular drow. Driders are unholy gods among their kind, pinnacles of poison and pain, conductors of terror and those who develop fresh hell for this endless war. Even to the scholars, these are truly monsters. Pathologically unknowable in their designs, biologically twisted away from anything close to a common race, and religiously fervent to the goddess that bestowed the transformation upon them.

With that in mind, let's return to your prompt, from the perspective of the town. A group of strangers arrive, with one of them battered and lamenting that drow have taken their friend. It's horrible, but no one is the least bit surprised: welcome to our lives, welcome to the struggle that plagues us every day. The town is immediately sympathetic. As the group recuperates, its clear they're not ordinary: their fighters, and they're not afraid to kill to get their friend back. Great! The town don't have warriors they want to risk on a stranger and what could be a suicide mission, but more power to you. If they're wealthy, they might even have resources to donate to the cause. They know how to point you in the right direction because, as we've said before, this isn't the first time this has happened. You might even get approached by some citizens, desperately asking you to look for their loved ones while you're there.

You leave and come back, in a way that is worse than any in the town could have ever dreamed. Townsfolk scream and flee from the drider as she walks past. By the time you've reached the town proper word has spread. Chaos is already burning through the town. Many citizens are fleeing town for the next several days: taking their children and living with a relative is far safer than waiting to see what is going on. You're met with drawn weapons and the defenders of the town, as many and as organized as they are possibly capable of in short notice. The hardest defenders have grim determination in their eyes, the youngest are filled with fear. Hell has finally come for their small village. They know they can't kill this monster from legends. They best can do is hold her off as long as possible, they'll most likely die buying just a few more seconds for the townsfolk to flee.

And then you inform them of the situation. How she is your friend and was transformed, but you brought her mind back. No one believes you. Why would they? Which is more likely: that a one-in-a-million drow was actually good, kidnapped, had a deeply unholy ritual forced upon her turning her into an inhuman monster of torture, was subdued by a group of strangers, and the power of their friendship drowned out the deep rooted evil inherent in this creature. Or, that drow kidnapped someone knowing their friends would rush to her rescue, a drider laid a trap for them and swung them all to her side with some sort of dark magic, and is playing innocent either as a joke, or just to see how stupid these elves are.

Even if they do believe you - and that's a nigh insurmountable if - what then? They can't possibly let this abomination of hatred linger in their town. If she loses her mind again? So she might? What happens then when the drider that they allowed into their midst eventually returns to its nature and looks slaughter and torture them all? And apparently if they defend themselves and attack this monster, you'll kill them.

The conversation quickly becomes, how can we get this group of people to leave, drider included, as soon as physically possible? The townsfolk are disinclined to fight: it's not a fight they could win even if it was just the drider, and they get nothing out of it that they couldn't get by just watching you leave. Her staying in the town, even with an amazing diplomacy, is simply unthinkable. Perhaps, perhaps you could convince them to allow her to stay if she agrees to be put in chains, in their jail, under continuous guard, if she also agrees to let them study and work with her to better fight drow and driders. But even that decision would sow discord among the community, possibly enough to splinter it to the point where people leave the town and never return. (Plus, that doesn't exactly sound like a situation you would be amenable to. )

If you leave the town immediately - the only decision I can see that doesn't involve a fight - I'd watch your back. Now that they know there's a drider roaming the area, they might inform the greatest hunters in the land and look to take her out quietly, just to be safe.

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That's how I would run it. I suppose this involves a couple assumptions of a lowish-powered world. If even a small town like this has the strength and resources to kill a drider, they might be more inclined to fight it (and you!) rather than let you leave.