I don't believe the ex girlfriend made up this account in his name and friended herself. If that was the case, there would be a reason behind it, like she wanted to cause problems in his marriage. If that were true, she would have then found a way to make sure wife saw the account. As it was, wife just accidentally happened to stumble upon it.
My theory: Husband didn't think wife would go searching for this account, because wife already knows his name on his "real" account. Why would he be dumb enough to use another form of his real name? Because lot of times guys just aren't smart enough to cover their tracks well. Trust me, I know!
OP, now that he knows you're onto him, he's going to try to cover his tracks well. Watch and see what he does with that account next. My guess is he'll delete it, because he's going to be worried that you'll figure out the password, and it's very likely he's got some messages in there that he doesn't want you to see.