The mutual friend is probably lying. It is a lot of bother to superimpose someone's head in a video. The mutual friend is just fishing to see if you had a sex life with your ex. If he was so nervous as you say and only lasted a few seconds, it is unlikely he would get the confidence to make a movie!!! A married guy complained to my ex that his wife was frigid, so as a stir, my ex told him we didn't have any sexual problems, in fact we had threesomes. He just did it as a joke, as we didn't have threesomes at all. But this lie caused a lot of trouble. People tell outragous lies about other people, for no apparent reason. Just stirring, they're bored maybe. It is very hurtful, the things some humans will do to others. On the other hand, your ex may be saying that you were involved in a threesome, because he is trying to give others the idea he is a big stud and you know he is not and he is embarrassed about that. I would avoid both him and the mutual friend.