If he has the
intention to reside in or
is residing in the state then it is legal. If you are worried about an ATF sting, then get some sort of documentation that he IS residing in the state. Lease utilities etc. Match the names
with his out of state DL. Make copies. Take a photo of the gun and serial number with a dated newspaper and tape measure/yardstick to show proper length. Give a copy to your lawyer. Basically the law is in a grey area.
He as to have the INTENTION of residing in the state, or be residing inthe state. Also check your local laws. I thought all PA transfers had to go through an FFL but I may be wrong on that detail.
But it can't hurt to CYA. I'm sure he will understand that all you are doing is CYA.
[edited to add:]
IF he has NO intention of residing in your state and or is NOT residing in your state it is against federal law. A drivers license is not what proves you live in a certain state. In FL you have 10 days to get your new license and 30 days to get your tags. You could very well have a lease and utility bills for your new state and not a DL. The grey area is the INTENTION part. and where I would get proof on paper or would deny the sale.