SPOILERS
Ok, I didn't actually think to give her the pink book instead, that's on me.
As for the rules, yes I do recheck, and on a second playthrough, I did notice that multiple characters would actually switch whether they were either a warrior or thief, if not both at once, and I couldn't figure out the difference until it went to the dungeon screen and said what was missing. One instance had someone walk up in warrior armor, a longsword, and a spell book, but it said I was missing the butt potion and wasted the boobs potion. I might be mistaken, but I also thought it said at one point that whips were melee weapons and ergo made for warrior multiclass, but it usually turns out to be 50/50 chance if it actually does. A lot of other times, I'd get a character that just looks like a mage, but they apparently counted as thieves even though they had no gear indications for it.