I'm going to encourage my best friend to dump his girlfriend.
Why are you trying to get him to leave her?
I do this because she makes him suffer. She also laughs at me behind me back.
This seems like a rough situation, and she doesn't seem very nice to him. Support you best friend though and let him know you are there for him.
I'm going to invite him to my house for dinner and a movie. Then I will ask him to dump her.
Well you can speak with him if that's what you'd like, but have you thought about what might happen if he doesn't want to break up with her? It would make sense to be supportive no matter what happens.