Provide firsthand assistance.
There are two primary slide-running techniques in use today, and both can be enhanced to work better for your customers with limited upper body and hand strength. The most common of these is what my friend Dave Spaulding, a master police trainer, calls the "saddle" technique. It's named so because the support hand sits like a saddle atop the slide, while the gun hand holds the frame. The thumb of that support hand is pointed toward the shooter. With a customer that's strong enough--and a light enough recoil spring light enough and, on older-style guns, mainspring --all that's necessary is for the support hand to retract the slide fully to the rear.
For someone with less upper body strength, this method can be enhanced in a couple steps. I first saw this demonstrated more than 30 years ago by Mike Plaxco, one of the great practical shooting champions of the day. It goes like this:
1: The customer's gun hand firmly grasps frame, trigger finger straight and above the triggerguard, while support hand "saddles" the slide, thumb toward shooter.
2: He or she then brings support arm's forearm flat into the chest.
3: Customer now uses gun-side shoulder and whole upper body to push gun hand and frame forward, while support hand firmly grasps the slide as the frame runs under it until slide has reached full retraction.
The second popular technique for pistol slide racking is the Israeli Method. The support hand grasp is different, with the thumb pointing downrange. Using all four fingers solidly planted on the side, the customer's thumb is firmly engaged on the other side of the "drumstick" of the thumb to the thumbprint. This provides optimum grasp of the slide.
The enhanced version of the Israeli Technique goes like this:
1: Customer's gun hand firmly grasps the frame, trigger finger is straight and above the triggerguard, while the support hand firmly grasps slide with thumb forward.
2: Next, he or she begins with the strong-hand side leg to the rear, slightly flexed, and then straightening that leg, driving the strong-side hip and arm forward--while simultaneously pulling back with the non-dominant hand.
3: Lastly, have your customer release the slide as soon as it reaches full extension.
This technique allowed both my daughters to positively run the slides of hammer-down 1911 pistols when they were each about eight years old and weighing well under 100 pounds. If it worked for them, it'll work for your customers.