Well, in the situation they are in, they couldn't afford the risk coming with the guy's proposal...
of course they could gag him, blindfold him and continue to go, and decide to kill him later on IF he proved a traitor... but still, in an enemy place, and very heavily outnumbered, the elves probably thought a goblin's life wasn't worth it.
Which opens the door to the "who is to judge which lives are worth it and which aren't" question, but that's one that doesn't really have answers, especially in war...
moral issues aside, I admit that at first sight I did find it funny especially when the two Resistance human soldiers are left to wonder whether to tell Than or not. XD