The problem is as stated, stemming from the disco not regaining the hammer correctly before the trigger is released to reset the hammer.
Confirm that the FCG is the correct pin size for the lower, then disco spring is the correct spring and installed large coils side down into the trigger slot, and the disco is in the correct timing as well. If the free gap between the hammer and disco with the trigger untouched is too great, then the disco will not correctly retain the hammer at trigger help back, and allows the hammer to be danced off the disco as the B/C come crashing home.
On disco timing, with the trigger untouched, pull the hammer back until you have the rear hammer sear and the disco sear as close to each other as you can. The free gap between the two should be in the .001 to .003 range (about the size of a human hair) and if he finds the gap to be too large, then metal is removed from the front/bottom of the disco where it forward maxes out on front/top of the trigger.
And here is the lower parts so he can figure out if he has the wrong spring installed for the disco.