Hum, well if you think it through.
You tell her now she'll flip at him then he'll tell her how you've played along so she'll end up flipping at both of you.
You don't tell her, he'll dump her once shes gone which will hurt like hell for her then when you finds out your out with him... unless its about 4-6 months after their break up she'll flip at you.
Although. He could dump her now saying 'i need to get over you i'm crap at long distance' so she'll hopefuly be over it by the time she moves then maybe won't be as hurt when she finds out you two are out. But most probably will still flip.
I think the best thing to do is for him to dump her now then you talk to her about a month after saying you have feelings for him would she mind. Of course shes going to mind but its up to her to what extent she takes it whether it'll be i never want to talk to you again or fine then bitches about you. 
Hope i've helped, a tad :/ lool. I'm sure everything will be fine, someones going to get hurt but you live and learn. Take care xx