Okay, how's this:
1: Make her Infected. Thus, she's a 4th level PC, but she can't control her powers (Let her take the control shape skill). When she can't control her change, You control her, to a Weretiger's aligment when shifted, and it's natural instincts (Seek prey, hunt, play with 'food').
2: Make her a true lycanthrope, 0-level character.
Edit: If you use #1, use examples like American Werewolf in London/Paris, and other references to the lycanthrope going nuts/losing control. That will likely mess up her concept, but tell her that's how it's going to be. She may change her mind.
I have an NPC true lycanthrope. The +3 ECL makes her about balanced with the rest of the party, after the combats I've tested.