My mom was sexually harassed in front of me

It upsets me just writing this but here it goes, today after a football game in my hometown my mom was sexually harassed along with other females. This scum who harassed her was drunk and very aggressive. He’s also one of her boyfriends friends. My mom was by the bed of a truck reaching for something when he snuck up from behind her. He slinked his nasty arm around her waist and thrusted against her backside while saying “yeah baby”. Her boyfriend did nothing,her friends did nothing, and I did nothing. I feel embarrassed that I wasn’t able to protect her from this pig and I feel ashamed that I didn’t speak up for her. I feel like I could’ve done more but I didn’t.