Staying idle when friends discussion is unpenetrable

Me an My good friend (not Yet girlfriend) were passing by the same places that we often visit. There she met a guy whom she met years ago. I knew this guy as we met him one more time few months ago. That was a brief conversation lasting few minutes only. But this time she started talking and it lasted for hours. for some time I was also participating in their gossip. But most of the times they were talking about some old stories that I cannot penetrate. and while talking he was touching her often without any reason.

What I am still thinking What One should do in my position at that time and after that time ?