What should I do? HE KISSED ME?!?

If she really is a good friend to you then you should tell her because he could be doing this with other girls. He obviously doesn't love her as much as she loves him. She needs to know because she deserves better. An you may like him but if he's willing to do this with you behind her back, then he'd be willing do it with somebody else behind yours. Be a good friend tell her the truth and support her because shes gonna need it Hope this helps xx