Ok. So your girlfriend saw someone she hooked up with in the past and HE was the one who initiatiated the over affection? Not her.

So if your girlfriend made an issue of it with him, potentially causing a scene, you would have found out hes an old hook up. Which is something she may not wish to reveal, as her sex life before you is really none of your business. But instead she just moved past it, nice and civil and things were cool until you raised issue about her "inappropriate" behaviour. People are allowed to speak to people they have previously slept with. Its polite if things ended on good terms, and both parties are comfortable. If you really have massive issue with it, then by all means end it. Its your issue if you don't trust her. And if she isn't cheating on you, then its not really fair on her either. I can totally understand her texts too. Shes tried to smooth it over and you have ignored her. You say its a red flag for you. Honestly i think it is probably one for her too. Blood boiling and stomach turning are pretty high emotions. And ill assume that her "friend" left sharpish, partially because he thought you might get hostile.

Good luck with it dude.