Talk to her act suspicious like say "ok so,have you know ever kissed a guy while we are dating?" She would most likely respond ,"no" then you would lie and say "I have " she would say really!! It's over and you say Jk just tell me the truth . Look straight into her eyes and to show her you love her
She obviously doesn't want to make a true commitment to you. She doesn't truly love you if she is doing this or if she did. You should really probably break up with her because you really deserve better.
Make sure it was her kissing him and not just him kissing her. Talk with her about it. She s no longer worthy of you. So its your choice to give her another chance or not. Dont do stupid things like beat the guy, pound him to the ground or getting a shotgun. It would only make things worse. If you want to break up with her, tell her you two can still be friends. And if she doesnt want to, then just let her go.
Talk to her if you love her then you guys will work everything out, and you will be okay. It will sit in the back of your mind for awhile, but you will learn to resolve it eventually. It could have just been in the moment thing, and she may not have wanted it to happen. So talk to her before you go and," dump her". I hope this helps :)