How do you even react to finding out your child did sexual things with your ex best friend

Me and my oldest child (22) were talking and we brought up my ex best friend. My ex husband had an affair with her which is why we are no longer friends. My son's dad died 6 months ago and we were just talking about everything, including the affair and he says out of nowhere I never told you this, but me and Ashley did things together. I asked when because me and her stopped being friends YEARS ago. He said when he was 13 meaning Ashley would have been 34. He told me Ashley had him go down on her but they never had sex. I got really upset and ask why he never told me and he told me to calm down and said I'm fine mom. I think. I don't THINK I'm that traumatized from it. Although maybe that's why I don't like going down on women. Either way I'm fine and if I have unresolved trauma I'm sure it will come out eventually and I'll go to therapy then.

I'm very upset he didn't tell me. Now I want to beat her ass which I'm not.... Because she moved... But this just got dropped on me and I don't even know how I'm supposed to react. This woman ruined my family in more ways than one and now I'm angry all over again.

@Kim idk if he can seeing as this was almost 10 years ago.