I'm going to tell my roommate that her friend has to leave.
Why do you think you need to say something to your roommate?
I don't like her friend being in the house.
They have the right to be there as well. You should respect your roommate and their guests better.
If I don't like my roommate's friend then they shouldn't be allowed over.
Your roommate has the right to have friends over whether you like them or not. You wouldn't like it if your roommate told you who you could invite over so don't do it to them.