that sounds really cool actually, a good situation for you and the dog, i hope that it works out... ditto the recommendation for classes... you may also want to invest in a belly band JIK he has some marking issues at your house (very big possibility for him to pee and /or mark in a new house, but if he is doing well in her house, it shouldn't take long for him to pick it up in yours

)
also, ditto on the just chilling out, try to keep yourself calm and just let him do his thing, let him come to you
keep us posted