My mother's boyfriend is her Power of Attorney, which he did behind her family's back. As her daughter, can I become her POA?

If you Mom is of sound mind, she can give POA to anyone she chooses of legal age and there is nothing you can really do.
If she is not mentally sound, you have a host of options.

Well your mom had to agree to the POA thing and her boyfriend right? Then if she's had a change of heart about it, there's probably something that can be done like go to the bank and talk to them. If she hasn't, then there's probably nothing short of taking it to court that can be done.