I am going to a music festival with my friend and her boyfriend is coming with us and I don't want him to.
Why? Is there a reason why you don't want him to go with you?
I hate him and I don't like the way he treats me.
You shouldn't exclude your friends boyfriend from the event. Imagine how bad it would feel to be excluded from an event that your significant other was going on?
Would he really feel bad if I exclude him from an event?
He might and so might your friend who is looking forward to spending time with you both. You should try to get along with him instead of excluding him from this social event.