I want my boyfriend to create more distance in his friendship with another girl
Why do you want your boyfriend to create distance with this girl?
I don't trust her and she is a bad influence on him
If you don't feel comfortable with your partner's friendship involving someone of the opposite sex then you should take it over. Be honest and upfront with them regarding your feelings. If not, then they may feel like you don't trust them.
What should I tell them? Tell them it feels sucks to see them together?
You can be polite but firm about it. Tell him how you feel about his friendship and how she's a bad influence on him. I'm sure your boyfriend will understand.