I would not forbid the relationship, because this is a guaranteed way to cause her to cement herself further to this boy. Bring him further into your family, and invite him over if they want to see each other.
Give her opportunities to bond with her friends. Invite them on weekend trips or shopping trips. Tell her that you will treat her and her friend to something she wants. Maybe have a game night at home one weekend night and either invite the boyfriend or a different friend.
My parents did this for me, and I finally realized what a jerk I was dating. Thank goodness!
They also sent me to college far away.