It is my understanding that the two best ways to slow the cyclic rate of a blowback system is to increase the stroke or increase the mass of the bolt group. Because of the mechanism it would be challenging to increase the stroke of the Colt blowback system. It would be possible with some machine work done to the bolt group, but the amount of slowing of the bolt group would be minimal compared to adding mass. The first link goes into that quite a bit. If you are limited by having the carbine buffer tube staked on, then the second link has information as what you can do with the carbine length receiver extension. The member "amphibian" in the second link has quite a bit of research that has has done on his website which is the second link.
While a stronger spring will ease the the energy of the buffer striking the end of the receiver extension, the addition energy that the stronger spring absorbs will drive to bolt back into battery, speeding up the cyclic rate. The flat wire spring does a much better job at having enough strength to strip the top round out of a full magazine while slowing the cyclic rate.
The "ramped" 9mm bolt has the same profile on the bottom of the bolt as a rifle caliber carrier on the bottom that cocks the hammer. Colt chose to not ramp the bolt to slow cyclic rate. The problem is the mechanical advantage puts extra stress on the hammer pin. The pin can break, but the gun will still run. The pin being broken makes the pin at an angle which will enlarge the hammer pin hole in the expensive registered receiver. Colt did start to use stainless steel pins as they were not as brittle as the standard steel pins. KNS makes fire control that ties the hammer pin to the trigger pin to try and relieve some of the stress. I would highly recommend the KNS pin system for any M16 RR. Even more so with a 9mm blowback system.
There is also a steel weight pinned into the back of the Colt bolt. Several manufacturers, including Colt, offers a replacement tungsten weight for the 9mm Colt blowback bolt. The tungsten weight can reduce the cyclic rate about as much as ramping the 9mm bolt increases cyclic rate. I had a configuration that would not run with a ramped bolt without a tungsten weight.
Good luck with slowing down your friends 9mm M16. I don't know what your friend has in his collection but the Colt 9mm blowback system is harder on the lower than the rifle caliber systems. Some manufacturers converted Colt SP-1 9mm carbines to M16 RRs. Colt did have a run of M16 SMGs just before the NFA registry cut off. Since there are only 100 M16 RRs in the registry and most of those were purchased by Police Departments, the Colt SMG guns are quite a collectors item. If your friend's gun is a SP-1 9mm, I would pull the magwell blocks and use the https://www.meanarms.com/products/detail/endomag-9mm
Endomag system. Then the gun could be used for lots of other calibers.