Wow, that's a tough one Jeremy.
How is it that the kids can't get into your bedroom but the friend can?
Unless you know the vet, I'd be surprised if he gave you any info on the details of the woman history of employment with him. That could open him up for a lawsuit.
If you accuse her and it's not her that wouldn't be good at all, especially with her living next to you. Plus you wouldn't catch the thief anyway.
I'd bait a trap for her and use a hidden camera.
Camera Cops Jail
... a few sugar pills,You guys are too kind.