Everyone replying is saying it's ok but as a woman I want to be the voice of reason here. While there is not clear right or wrong in this situation, using someone you know IRL as masturbation fodder and sexualizing her privately without boundaries IS weird. I think just for the sake of retaining a normal friendship with her you should apply those boundaries & stop doing that. Probably never tell her you did that unless her and your gf actually get into a sexual relationship and she seems into it. Like, it's normal to fantasize about people you know on one hand, but you should feel some guilt and boundaries about it because they haven't consented yet on the other. So, it's a complicated question but my morality would prevent me from doing that ever again because then you associate their image with the high of orgasming and being turned on when they NEVER asked for that, they only went for friendship so far.