And how do you know this?
Because it makes sense. Which ever is the fake identity, both of them are related by as you mentioned being friends on Facebook. He made a fake account for whatever reason and friended it.
Even if they are two different people, they still at least have to know one another in order to friend each other on facebook. There's not that much of a big chance that he's a real handicap to go ahead and completely blame the other guy he knows by using his identity to commit several crimes.