Well, there are a couple questions to understand more.
Did he transfer the guns to you before he left to Europe thru a FFL?
If he did, then you will need to go thru a FFL to send them and he will need to do form 4473 to obtain them.
If he did not, there’s legalities here that you shouldn’t post on this forum.
My suggestion is to call him, ask him how he wants to get his guns back and have him do the research in order for you to be able to send them. In addition, the cost to ship them. Firearm shipments require min. 2 day delivery. He may not even want them back….10 years is a long time. But ultimately, this is him, as the gun owners responsibility to understand how to obtain them.
Road trip might be on the horizon or you can ship them via UPS or FedEx, but verify if FFL is required to complete the hand off in Arizona.
I’m a FFL and I know I tried to buy a gun (personal use) and they would not ship to WA. It was just the choice of the FFL I was in. I’ve since received and transferred guns from and have sent guns to FFLs in Arizona.