Hi,
Did she say when your boyfriend sexually assaulted her? I would think this would be of importance if he actually did. I think getting all the facts from both parties would allow you to find out the truth. If he actually did assault her, she has a point, although a bit immature in the way she has dealt with it. Perhaps she could have said something like "Mom, I need to tell you something that happened that I've been keeping from you".

Confronting your boyfriend and seeking the truth seems like the only way to go here. Once you have the truth, you can make a better decision.