Relationship advice or help please?

In a 16-17 year old relationship that has been going on for 2 years, is it normal for the guy to hang out with a girl..alone, without me, when she likes him? He doesnt see it but its obvious she likes him because shes been obsessed with hanging out with him for a while now, i barley know this girl and apperently theyre really close friends and he helps her with her problems ? Theyve planned on hanging out tomorrow ? I need advice or just any opinions on this because I already told him how i feel and he said he should be allowed to hangout with his friends...i trust him, but i dont trust her and im just nervous about this situation=/