Monk player would like to betray party - how to balance?
Hello All,
I'm a fairly inexperienced DM, about to end my first campaign (DnD 5e, 2014 ruleset). One of my players asked me if they could betray the party and turn against the other PCs.
I'd like to allow this, mainly because such plot was always a possibility in the story of the campaign, would fit the overarching theme, and multiple times it was offered directly or indirectly to different players. However, I'm afraid that this would end up in a fairly quick TPK.
This would take place during the final boss fight, so of course the betraying player plus the boss would be the enemies (plus trash mobs, AoE effects). The party will be lvl 10, and the betraying PC would be an Open Hand Monk, against 4 other players. There's a possibility that the party will be helped by NPCs (currently a paladin- and rogue-type NPCs are candidates, depending on what the players do).
Should I allow it? Is there a way to balance this, so that despite the betrayal the party will still have a chance, but I also don't completely negate the abilities of the betraying PC?