Most Helpful Girl
As it should make you feel like that, it's not right. Not only does your girlfriend need to understand boundaries and she should've "shut the guy down" herself (to quote @Iron_Man because I agree with him) but also you could talk to him as well and make it clear to him. Also try talking to your girlfriend about boundaries and what should/shouldn't you guys be doing and what's allowed/not allowed because she either would have no problem cheating or just doesn't get how relationships fully work.
Most Helpful Guy
No good you need to shut that dude down right away that's disrespectful