I want my partner to cut communication with a specific person. Is that too demanding?
Why do you want your partner to cut communication with someone?
This person is a bad influence on her and I think she's too dependent on him, but I don't have a deep understanding of their relationship
Unless they are known to be dangerous you shouldn't be able to tell your partner who to talk to. Would you like it if she told you not to talk to your friends?
Of course not but she doesn't understand that he's a bad influence on her.
Well, don't do something towards her that you wouldn't appreciate being done to you.