Can someone point me in the direction where it says in the rules that ability focus a monster feat can be used to enhance a witches hex.
I have found some threads where some posters say it can be done, but I can't find one that has pointed to the rule in which this is allowed.
The text just says that some characters might qualify for monster feats. Though I am not particularly sure if it is allowed, Hexes are a class ability, not a special attack (not 100% sure on this), so my initial response is not to allow a player to take it, especially since the hex DC's are going to be higher than the spell DC's anyway. The short answer is just ask your GM.
I would tread very carefully here due to potential game balance issues. I don't believe the hexes were intended to be used with monster feats and so there may be some unintended synergies that could be exploited.
I've avoided this with my witch mostly because I feel he's plenty powerful already and don't want the GM to feel I'm "power gaming" any more than I get kidded about already.
It really is a slippery slope and I feel allowing this feat for PC's is just asking for all kinds of trouble. Clerics with ability focus on the channel, an oracle with a higher DC on revelations (awesome display *cringe*), an alchemist with higher DC bombs, etc. Yeah the more I think about this the worse it gets.