Please, please, please help me with this huge problem?

My friend and I have been mates for a long time now, i'm 19 and he's 22, he said how would I feel about a , he has a girlfriend, and obviously that's what I thought he meant, so unknowningly I said yeah i'm up for anything, when we got to his he said we'll do it now and i said where is she then, he said in the wardrobe, puzzled....i said right! he said come up in a min, i said ok, i could here them starting, he shouted for me, i took of all my clothes, then he was there on the bed doing a doll, does he love me and want me, but using doll as decoy, should i tell his gf????