All of the legal issues are easily solved by a power of attorney. I've spent more than a few years in Afghanistan and Iraq and have had to use them regularly for vehicle registrations, selling vehicles, etc.
With all of that said, you need to talk to your buddy and attempt to talk him into waiting to return to make the purchase. I spent many hours looking at trucks online while deployed and understand the desire to make the purchase immediately, but there is zero benefit to doing so. It is just more payments he makes while not being able to use it and more depreciation.
Sometimes things on dealers websites don't show things that you would immediately dislike in person.
Talk him into waiting.