I don't know how to approach a friend about her son

I am friends with my neighbor and her 3 kids.  Her oldest son who is 11 years old loves being around me and playing football or games on our phones.

I've noticed that around certain boys he seems to do a lot of joking around sexually.  Today I believe he took it too far with his sexual joking because he did a couple things which were inappropriate that made the other boy we were playing football with feel uncomfortable.

I told the boy to go tell my neighbor what her son did to him, at first he said he would then after a few minutes he decided it wasn't worth telling my neighbor about what happened.

I want to approach my neighbor about what happened but where I didn't actually see the stuff happen due to my neighbors son intentionally doing the stuff when my back was turned to them and the boy it happened to saying he wouldn't say anything I don't don't know how to approach my neighbor without making myself look bad.