Just got my copy and installed it, I'd played the demo so I knew what to expect. I went through a few battles before noticing something was off. 
 
Bethany isn't acting in combat, she'll take out her staff and just sit there even while being attacked. I've had to take control of her so my party doesn't have a completely useless mage sitting around. 
 
I've tried everything I can think of to fix this: reloading the save file, reseting the game, fiddling with her tactics, nothing seems to work.

Did you try starting a new game.

Tried looking at her tactics to see that she has any? Or is that even included in the console version?

The place to visit (if you haven't already) and post about this is probably their forums. That one's specifically a PS3 user discussion forum on technical issues, so you might also be able to figure out if there's an easy fix. 
 
EDIT: I changed the link.

Make sure she isn't set to passive.  If she isn't, try setting her to passive, playing for a second, and then changing her back to default?