My fiancés son who lives with her he is 22 went to a party at his dads house. During the party he walked into the kitchen and saw his fathers wife kissing a neighbor. He doesn't know what to do. He told his mother and she said to leave it alone as maybe they were just fooling around during a party atmosphere. He said what he saw was not fooling around. His father has two young children with his current wife and it's a tough situation for my fiancés son. He doesn't know if he should tell his father or not. I am not sure what to advise her as she asked for my opinion.

He should drop it.
Just for starters, his own mother said to leave it alone, and she knows more than we do about the situation.
Second, there are just thousands of cases of 'kisses' that do indeed have no basis for concern. There are also thousands of relationships that have been ruined by meddling over something that wasn't what it seemed.
Third - he's 22, an adult. He needs to MYOB.

I imagine it was shock to him to walk in on his dad's wife kissing a neighbor, but maybe best to do nothing rather than start a big drama with so few facts.

If his own mom said to drop it, then that's what he should do.

Thanks for some good advise.I hope he can leave it alone.

I agree, just drop it,

Leave it be. If his own mother said leave it alone, it should not bother him. Why start mess over this. He needs to mind his own business.