Suggest you do NOT reduce the strength of the recoil spring because that will just lead to other problems. If the, 'support hand over the top of the slide's rear - w/ the palm down
' method isn't working, then, try using the slingshot method by pinching the FRONT of the slide before pulling it backwards.
Two years ago I broke both my wrists at the same time; one year ago I broke both pinkies, together; so, I definitely have sympathy and a sense of appreciation for what your friend is going through. Pinching the front of the slide worked well for me. So did catching a front corner of the slide on the edge of a shooting bench. You can, even, use the hand over top method in conjunction with pressing the muzzle against the edge of a post or table - Any of these methods will work.
I do, however, strongly recommend shooting the pistol dry rather than attempting to rack the slide on a loaded chamber. No way would I use either of these expedient racking methods if there were a round in the chamber! (Cause bad things could happen.)
Remember to keep the muzzle pointed in a safe direction; (Rule #2!) and do NOT place your fingers in front of the muzzle at anytime. One other thing: This might be a lot easier to do with the square muzzle configuration on a Glock than the more narrower and more rounded profile of the Sigma; so, you're going to have to experiment and practice for awhile with an EMPTY pistol - OK!