How to fix bad reputation

[An tAthair Ted]

My friend and I have been playing co-op for the past few days. He has killed tons of NPCs, which has brought his reputation down to -19 or so. Now wherever he goes initially friendly NPCs begin attacking him. Is there anything that can be done at this point to fix his reputation? Thanks in advance.

[Wovan]

Things said, cannot be taken. Things done, cannot be undone... unless you save&load or pay the demon a hefty price.

There's a demon lord in the homestead that can increase your reputation, He's like 2-3 last rooms to unlock so it's past halfway through the game.

Until you unlocked him, your friend just gotta stay out of town while you do the talking. If it's a personal NPC, just bride them with gold. Or if you're lucky, enough [good deed] quest will bring your rep barely back to good. Though 19's a lot for a comeback.

Edit: Charisma might also help by increasing your friendship value a bit.

[FrauBlake]

Finishing quests increases reputation. (I'm not sure if all do though.)

Over-paying when buying or giving stuff away for free when selling also increase reputation, but only with the trader that gets overpayed.

Since trading is impossible when one gets attacked immediately, the only chance is to finish quests (in a location which has a different alignment) or, as kvang145 said, using the demon in the homestead.

And, as also mentioned, increasing Charisma might help although I'm not 100% sure.