According to what you wrote, I think he is using you...I mean, If a guy would really love someone, he would be with that person not a GF, who is far away. And I think, you should respect yourself and his GF and cut the relationship, or ask him to choose. If he really loves you, he will choose you..if not, then it is good you asked him, coz your time is too precious to waste it on someone, who doesnt worth it.
p.s. I have distant relationships, and my BF is far away from me, I am sure in him, he is very faithful etc, but I feel really sorry for the GF of ur friend right now, coz I can imagine what pain her BF is giving to her right now by being with you...