I have teenage nephews and this sounds like something stupis they'd do. They do stuff that has me scratching my head all the time, wondering if their Mum may have dropped them on their heads when they were little or something, because I can't figure out WHAT they're thinking.

I think this boy was just "showing off" to his mates. I don't think it was for sexual gratification or anything disgusting like that, just that sex is "naughty" and he thinks it's funny. I don't know if it's worth mentioning to his Mum at this stage, maybe give him the benefit of the doubt that it was just a one off and he was being a "boy" and if he does anything like that again, then say something?

Evan sounds like a nice honest boy, I'm sure he'll tell you if this boy does anything else.

HTH