Pretty much everything AR is commanding huge prices right now, regardless of vintage. But if the father of your friend wants an Vietnam era look-a-like, I'm not aware of any manufacturers who are currently building M16A1 clones. You can piece one together yourself, but it's gonna take a little work hunting down the right components.
I think the best bet is to look through the various gun auction sights for a Colt SP1. The SP-1's were early civilain variants of the M16 and should be dead ringers for what the father saw in the service.
"Si vis pacem, para bellum" - If you want peace, prepare for war.