I want to ask my girlfriend to stop being friends with someone
Why do you want her to stop being friends with someone?
I really don't like that guy.
I understand, but it's not nice to tell others what to do. You should respect your gf's decisions.
Why can't I tell her to stop?
If they are friends you need to respect that she has her own friend circle. If you are insecure and suspect she might be cheating then it is reasonable to worry about it. You don't get to control her.