For starter, read the manual.
Once you have gone threw the manual, then it's time to start checking your lower receiver build to confirm that everything is install correct.
After you have checked/tested the FCG, then it's time to check the hammer and firing pin.
On the hammer, it may be a modified m-16 with the auto tab ground off. If this is the case, you may need to round the top edge to be like that of the semi AR-15. Also, check the tail of the hammer for contact on the disconnector. If your hammer is binding at the top round, then the tail will bind too, due to the hammer sitting closer to the carrier than on a colt.
On the firing pin, check the diameter of the retaining ring. It may be a M-16 firing pin, and with out the hooded carrier, the hammer may be catching the larger than semi, retaining ring.
And, for the sake of your friend, stop building up other peoples rifles until you full understand the AR system. It's one thing to have a problem on your own rifle, but when you start having problems on others, they lose confidence in the rifle and You!!!!!