Yeah, that's the worst part right now...
Worst case, she didn't realize she was forbidding most of basic Monk features with armor (in which case Monk may be a tad too tactical for her), you should then probably allow a respec.
Best case she actually have a specific idea in mind.
- Either using armor just for the first levels, bearing playing like a regular Fighter, then removing armor.
- Or actually just wanting a bit of Monk features (Dodge as bonus action, Deflect Arrows, Long Death's THP) with just a bit of Ki tacked on top of a Fighter, which can work well too.
In any case, I agree she should know which class she wants to be the main one, and get it to 5 before character level 8 worst case.
Bolded part: that is simply untrue in many situations.
When you talk about skirmishing as "hit and run", if and when ranged attack from more than 150 feet afar in a place with surrounding covers is a thing... Then yeah Rogues will be better, because then can Hide & Attack in same turn, rinse and repeat until time to bail comes.
How often does that hold true though? If you're looking to harass a bandit camp in a forest, yeah no problem.
If it's about trying to silence one single guard holding the entrance to a fort or picking stray targets that expose themselves upon a wall, it can work.
In any decently organized place, Rogue's hide and seek death game can easily be made rather difficult if not completely folded.
And if they try to get inside to play with their dagger, they'd have first to get past the walls / doors / locks. On the latter part, the right expertise is usually enough. On everything else? Reliable Talent will come in handy, but that's a level 11. And even then, as far as climbing goes, as far as creating cover/distraction goes, you'ill still need equipment, time, plan, and a bit of luck.
Only Assassins (impersonate a guard), Thief (climbing speed) will fare well. Arcane Trickster may find some ways, covering climing with a Fog Cloud, or using a Misty Step later, or creating a distraction with an illusion... Still requires planning and some luck.
Monks? Around the time the aforementioned get real tools for the job, climbing up 30 feet of wall is no problem. Shadow and 4E are particularly good in this.
You don't have ONE but SEVERAL guards to disable? A risky but potent bet could be full nova of Stunning Strike on both.
As for scout/spy (which is not at all a tertiary role in my view, at least if you want party to survive intelligent hostile factions, but taht's another topic), Monks can easily be built in the top 3.
They don't have Expertise unless they happen to be Human or Half-Elf, granted.
At least two of the archetypes (Shadow, 4E) do get extremely good abilities for sneaking around.
Their natural Wisdom makes them an ideal candidate for Observant.
Their "permanent Comprehend Languages" makes them a heck of a spy late game, *especially* when tacked on Observant.
There is nothing a Monk can do that a caster couldn't make, probably better, with the right spells/rituals.
There are few things a Monk can do that a Rogue, *dedicated to that*, could not best, choosing rightly the Expertise and feats. But it would require that "build investment".
The only thing I see would be "permanent Dashing" (which is not possible normally anyways).
Monks are naturally good at scouting/spying as they get into more levels.
Monks become more tanky than Barbarians and Paladins in the last third of progression.
In a party that has several casters anyways, that could buff the martials with any kind of buffs they could need, that would probably feel irrelevant. In most parties, casters have largely their hands full with buffing one particular guy while holding a control spell on enemies and taking care of emergency heals.