As others have said, if you want one close to what your friend's father carried, the Colt SP1 would be the easiest way to go. Then again, he might be able to save some money by building a clone, though it might take time given the current climate. If neither of you have built an AR, Brownells has a nice video series clearly showing just about every step, and the requisite tools.
There are a lot of different variations to the M16 series during that time period. When did your friend's father serve? What branch of the armed forces? If he decides to go the build route, you might as well dial in on what he most likely carried. For example, the Army for the most part used the XM16E1 early on through about '67, which was super ceded in increasing numbers by the M16A1 from there on. On the other hand, the Air Force and I believe the Navy and Marine Corps typically
used the M16 (no forward assist or brass deflector, IIRC). And there are slight variations for each of these 3 main versions, particularly in the XM16E1. It just scratches the surface, and forgive me, my memory is playing tricks with me at the moment.
I built mine to represent a very early XM16E1, which shared some components with the early M16 such as the 3 prong flash suppressor and dimpled receiver parts (but it had the forward assist). So in effect, the XM16E1 was a transitional model.
In the end, it was great fun studying the different versions and tracking down all the parts. I cheated a little and went with matching NoDak Spud upper and lower receivers. The dimpled controls come from heat-n-beat. Other parts that I bought new I ended up treating with petroleum jelly to turn them from black to gray (the trigger and charging handle come to mind. It's not exact, as the barrel is a 1x9 twist from Del-Ton, and the bore is not hard chromed. But it looks good and I have fun with it, and that's what counts.