Oh Sweetie,
Please don't think this was your fault! It wasn't!! You did not ask for this. That guy had NO right to touch you in any way.
YES, this is sexual assault. Anytime anyone touches someone without that persons consent, it is sexual assault. It isn't sexual harrassment, as sexual harrassment is if a person says sexual comments to you. He went WAY beyond that. He touched you. That's sexual assault.
IMHO, I would tell your friend, I would also tell your husband. You need to tell your husband right away. The longer you go without telling him, he may think that you are trying to hide something. So, as hard as it will be, he needs to know. Explain to him that you did NOT encourage the guy whatsoever. I'm sure your husband will be really mad at the guy, but he doesn't have any reason to be mad at you. You did NOTHING wrong.
Please don't blame yourself or feel ashamed. It really was out of your control. Even though your friend couldn't have stopped what happened, they need to know about it too. Then, they can make the decision on whether letting this guy continue to hang around them or not. Hopefully, they will do the right thing, and tell the guy to get lost.
I am so glad that you felt comfortable enough to tell us about this. It shows that you are a strong person. You should put your picture back and stand up and be proud. You did NOTHING wrong!!
Hugs,
Linda