This happens to me sometimes. If the game I played is with a party of some of my friends, it sometimes goes under their name. Like I played a game with 2 other friends, and later we all went in the theater, but i wasn't the party leader, because i didn'tt have the best connection. So when i took the screenshot, the author name was the name of the party leader, not mine. All I can suggest is that you have your friend tell Bungie that he didn't take the screenshot, and that you did. I'm sure Bungie will believe you. Hope this helps!
Also, if you want, add me on xbox live...my gamertag is dust fingerz106