There are men who are predators dear. They seek women with daughters. It absolutely happens and it sounds like you've found one. I would immediately break up with him because I'd always have doubts in my mind now. And your daughter's protection is the most important thing. There are plenty of men to love in this world and you only have a one, two (or how many kids you have).
Drinking is NO excuse. NONE. And if he drinks until he has poor judgment, even more reason to find someone safe to have around your child. good luck
It doesn't sound like she was lying. I would ex the boyfriend right away. It doesn't matter if he is subsequently sorry, he did something that he wanted to do and it was a very inappropriate thing for him to do. You said "she does not remember the conversation details (neither does he... it was a weekend and he was drinking, ...)" as though his drinking makes everything excusable? I think if someone is a drinker AND if he has something wrong that he wants to do, he is sooner or later going to be drinking and want to do that thing and won't stop himself from trying. You're going along with a sword of Damocles hanging over your life (and especially your daughter's) if you don't cut ties with him. Please take this seriously.
WOW! Yes you should be concerned and very VERY alert. Thank God your daughter felt comfortable enough to talk to you about this. Its our jobs as parents to keep our kids safe. Don't wait until its to late to get this figured out. She also may be going through more than she is willing to say?
Wow. This is a difficult situation. I think that you really have no choice but to sever your relationship with your boyfriend. Even if your daughter has a history of lying or grossly exaggerating situations, this situation could come back to haunt you if your daughter ever officially accuses your boyfriend of sexual abuse and it was discovered that you knew about this event. Not only could you be charged with a crime but your daughter could be permanently removed from your care.
It might be a good idea if you sought mental health therapy services for your daughter to help explore this issue even further. If she was in fact abused by her boyfriend, this could be discovered in therapy sessions. If your daughter is fabricating lies against your boyfriend, This also can be discovered through counseling sessions. But if I were you, I would definitely play it safe by severing your relationship with her boyfriend until this issue is better explored by mental health professional.