If I were you, I would go back in to his accout and unblock this girl. Then I would check it every singel day to see what they are doing. She is the cheese and he is the mouse. Otherwise, how will you ever know what his true intensions were going to be with her?
Obveously, don't let on to him that you are spying, Good luck!
"Your value doesn't decrease based on someone's inability to see your worth." -Unknown Wise Person
I wouldn't re-friend the person. This sounds like a can-o-worms that's just going to make you feel worse in the long run.
What it REALLY worth addressing is the paranoia that we all know so well, unfortunately. Has he never come clean, or do you really feel that this is just a lingering after-affect of the affair. I want to lend perspective, but I don't know how close to the trustworthy line your H dances.
Even if it wasn't her...what the Hell is he doing being friends with a 15yr old girl?
Also, if he "has always been transparent" then why don't you already know about her if it is indeed his XAP? I doubt she would send the request and he accept and not communicate at all...