Since I didn't see the Frizzen or the Spring before he did whatever he did, and I didn't see what he did to lighten the spring, I can't answer your question. If his lock was like many, the Frizzen may not have opened at all, when fired.
Rebounding is related to the fit of the frizzen to the pivot screw/pin. Most modern locks have a Frizzen BRIDLE that allows a screw/pin to be driven into the bridle from the backside of the Lock Plate. Some few locks are still offered that have NO Bridle, and rely on the strength of the Screw/pin that is screwed into the Lockplate from the outside of the frizzen.
If those pin/screws are Loose, or too tight, you have problems with the frizzen movement, regardless of how much tension is applied by the Frizzen spring. If the frizzen is poorly fitted to the bridle so that there is sideways "SLOP" or play between the side of the frizzen, and the lockplate, the frizzen can flop around, regardless of the amount of Tension of the Frizzen Spring, moving both back and forward, and side to side.
I have seen pivot pins that were both rusted and bent causing frizzens not to move correctly. I have seen pivot screws that were loose, and only needed to be turned in, oiled, and a few small burrs removed between the lock plate and the inside edge of the frizzen to get the frizzen working properly. I examined one older lock that had lots of play in the frizzen, side to side, because someone had replaced the original factory frizzen with another- reason unknown-- that simply was too small in width for the bridle. It would have required shims between the frizzen and the lock plate, and another between the frizzen and bridle to keep it centered inside the bridle.
I have seen a couple of locks where the hole for the pivot pin/screw was too large from the size of the pin/screw, and there was excess movement both sideways, and up and down as the frizzen was handled, so that the frizzen did not seal the flash pan well at all. Both frizzens flopped around alot, and even occasionally bound up because they twisted when they moved, and any dirt that got onto the pin allowed the frizzen to turn a bit sideways, and rub against the pin front and back. When those pins were removed, you could see where edges of the frizzen were rubbing grooves in the pin.
So, there are lots of reasons for the frizzen to "flop", that have nothing to do with frizzen spring tension.
I will repeat this again. The SOLE job of any frizzen spring is to keep the frizzen closed, when your point the muzzle of the gun Down, vertically, towards the ground. And this is only needed to keep your flash powder in the flash pan.
Once that idea sinks in, the entire reason for changing how the spring tension is transmitted to the frizzen becomes intuitive.
The problem with "solutions" is that there are a variety of badly designed flintlocks out there. No one solution works on all of them. The " Fix" has to be designed to deal with design problems for the individual lock. Since the Cock and its flint are what open the frizzen, some of the design problems occur in the length or height of the cock, sometimes in the relative distance from the cock's tumbler pivot, to the flash pan, compared to the height of the pan, and other locks have frizzens with too much length and resulting weight behind the pivot point.
In some, you find that the toe is little more than a lump of metal forward of the pivot point, and that over the years that the lock has been made, casting molds have been changed, and the Lump has become bigger and bigger. Other than removing casting seam burrs, lock companies do little to correct the shape, size, and balance of the toe in relation to the rest of the frizzen.
The Golden Mean Proportion, and a pair of dividers is a great way to figure out where the problems lie in a given lock.
