How can you get someone to dump their girlfriend?

Answer 1

Tell each other things that aren't true. For example:

You tell the girlfriend when she is alone that you saw her boyfriend with another girl.

Or you tell the boyfriend you saw her making out with another guy.

Say stuff like that too each other, and then when they blame it on you, blame it on somebody else.

Or even start a rumor, so that it finally gets to one of them.