His mom thinks his dad is cheating. How do you tell her that he’s not?

“I’m 17 and I guess my mom found a bra in the living room that my girlfriend left when she came over. I pretended that I had never seen it, so later I heard my mom scream at dad and accuse him of cheating. I don’t know what to say because my mom is very traditional and thinks I’m still a virgin, but obviously I don’t want my dad taking the fall for it.
What’s the best thing I can do?”