I (34F) went out for dinner with my coworker (40F). We had dinner and drinks and ended up going to a bar and met up with her boyfriend (38M). We all had a lot to drink and while we all were dancing together he turned to me and pulled me towards him and grabbed my ass. I immediately pushed him away and asked him wth he was doing. He said something stupid about us having a threesome and I just told him he was an idiot, but I didn't want to make a scene so I didn't tell his girlfriend about it and we all ended up having a great time after that. He didn't try to come on to me efter that either and I made sure that I didn't get close enough for him to be able to do it. Now I feel bad for not telling her as I feel that we really connected on the dinner we had and spoke a lot about ourselves and I feel like she might be a friend I would have outside of work too. However, her boyfriend works with us too. And he and I work really close together as we're both part of management. He has been a jerk to her in their relationship but I know that this information will not make her leave him. I'm afraid I'd tell her about what happened and end up with her being uncomfortable whenever her boyfriend and I need to work together, he'll be pissed of because I told his girlfriend and I'll just be caught in the middle. So, AITAH for not telling her about her boyfriend?