Problem is in his lower,

Start with having him pull the charging handle all the way back, and confirm that the face of the bolt is stopping 1/8" to 1/4" in front of the back edge of the ejection port window, and during the CH pull, the charging handle does not increase in the needed tension to pull it back from about half way back, to all the way back.

If the Bolt is stopping way short, then suspect that either the buffer is the wrong one in play,  too long of a butt stock screw for the stock he is running, and it's protruding into the back of the receiver void and stopping the buffer from coming all the way back to touch the back of the receiver extension back wall, or he has the wrong buffer spring is play (after market, with too many spring coils isntead.)
Note, remove the buffer and spring, and make sure that the end coils of the spring are not scrapping on either the inside of the tube, or down the sides of the buffer.  If needed, use a file to remove the protruding sections of of the end of spring coil burs.

Next, hold the trigger back, and do the full pull again.  Here, he should feel the disco being pushed back on it spring as the hammer is cocked to the disco, but once the hammer cocks onto the disco, the trigger should return freely all the way back.  Also, shotgun the upper open, and take a good look at the disco and trigger tail for signs that hammer tail is tagging them hard or that the tail of the hammer is binding up on the back hook of the disco (instead of coming down to touch the tail of the disco only instead).  If there is wicked white marks or dents on the tail of the disco/tail of the hammer, then the tail of the hammer may need to be shortened so it now wedging/binding between the tail of the trigger and the bottom of the carrier when the B/C is stroking.

Lastly, double check the mag release button and make sure that the threaded section tip of the mag catch assembly is flush with the face of the mag release button.

To add, may sure the FCG springs are installed correctly, including the disco spring large side down into the trigger tail slot.  Hence if the disc spring is upside down, may be enough binding on it/ the disco trying to lower to be enough to wedge the carrier up on the way back.

Not a spring problem, but either the receiver extension depth out of spec, or the buffer too short instead.
Hence the recoil spring is not a  limiting factor in the distances that the buffer travels rearward (should never get in a spring coil bind up problem), and it the lenght of the buffer against the depth of the tube that prevents the back of the key from crashing into the lower receiver.

So with the buffer and recoil spring removed, measure from the end of the receiver extension tube to the bottom of it void,  and the lenght of the buffer.
The depth of the tube should be 9.625, and buffer 5.25".

Also, where did the parts kit to build the lower come from, since with the spring having 39 coils, the spring is  screaming air soft knock off part, and may have other air soft parts in play as well in the parts kit..