Try to get all of you in the same room. Then, tell the 'other girlfriend' that he was dating you. Then the other girl will get angry and then she will ask who is his girlfriend. The boy will get confused and say both. Then that's when you and the other girl dump him.
Chuck him and find a new boyfriend.
ask him upfront
you catch him out.
If you can forgive your sister, you can forgive your boyfriend. It doesn't necessarily mean that you should stay with your boyfriend. Cheating is cheating and if that is not acceptable to you, you should find someone who will respect that wish.
by asking your best friend to spy on him
No it is not cheating on your bf. As long as nothing physical happens your not cheating.Talk with your boyfriend about how much you're allowed to do with other guys without cheating him. Everyone has different ideas of what cheating is and what matters is that you two find a common ground and agree not to break that.
Find a new boyfriend. If he did it once and you forgive him. He'll do it again and expect the same.
there probably cheating on you
If you have a girlfriend and a boyfriend, then you are cheating on both of them.
Then do nothing and be glad he is no longer your boyfriend.
Answer: No. Answer: I don't see how this would be possible - except if you catch a disease from your boyfriend.
Answer If you know for sure that your new bf is cheating then perhaps it's time to drop him and find another boyfriend, who needs the hassel of a guy who lies from the seat of his pants.