More advice, if he's a true friend and you want to keep him as a friend then give him the extra $50. Sounds like he was desperate and the gun is well worth $400.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek