Never let other guys touch your girl. Females are biologically programmed to fall for guys who make repeated physical contact. This is made worse if he is feeding her emotional 'feel good' conversation. There is hard science behind this, btw, its not just common opinion.
I think your only hope of salvaging this is to go outright CaveMan on her & tell her if she keeps seeing this guy, you will dump her.
You will likely breakup anyway, but at least you'll have your pride intact when you do. And you'll have learned a valuable lesson.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh