I'm going to tell the girlfriend to not talk about her friend
Why do you want to stop your girlfriend from talking about her friend?
I just want to save her from her friend's misunderstandings.
Your girlfriend sounds like she has a lot to say about the friend which I suppose is coming off as annoying. You have the right to tell her you don't want to listen to this friend's nonsense. However, doing so might make you look less caring to your girlfriend which might cause tension in the relationship.
It will only be a temporary change until things get better. The point is to make her feel a lot better if I agree to her please.
You are on the right track, but it might be better for you to explain what is bothering you about her friend rather than forbidding her to talk to her.  It will make her more likely to listen.