Migration was only automatic for the initial time (and several agreements had to be agreed to prior to it even then). If your friend's account was Migrated outside of the one-time automatic Migration back on September 30th, then it was manually initiated on the Bungie.net app or website by a person; it was not done automatically. If your friend didn't do it, then someone else with access to your friend's account did it, and your friend should speak with people they know. Once the Migration is initiated, the Blizzard save data completely overwrites the Steam save data that might have already been in place. This means your friend's character(s) and the like are non-recoverable on Steam as they simply no longer exist. The Blizzard save data became the Steam save data, and the Steam save data that was there went into the void. So, unfortunately, no, there is no way to fix it.