A 3" Ruger SP101 with .38s is very soft recoiling.
A 4" K-frame is as sweet shooting a gun as you are ever likely to find. A Model 10 is tough, rugged, and bone simple to operate. The Model 15 has the same virtues, of course, only with adjustable sights. Adjustable sights aren't as tough as the fixed sights on a Model 10, but are better suited to target shooting.
Personally, I think the L-frames and GP100s are just a tad heavy for folks who may be lacking in hand and arm strength.
If they cannot rack the slide on a semi-auto, they will probably need to shoot the revolvers single action. Make sure they are comfortable reaching the hammer with whatever they choose, if that is the case.
I had a similar situation with an older couple who took basic pistol (separately). They came back for another session together and went through a number of revolvers including a GP100, two SP101s in .357 and .327, a Model 19, and a Model 36 Airweight.
Hand strength and arthritis was an issue for both of them, and neither one could rack the slide. Both also had trouble shooting double action.
They ended up buying a 3" SP101 in .357 (and will use .38's) neither of them cared for the muzzle blast of the .327.
The Model 19 has a sweet trigger and the woman did like the DA trigger pull but she just couldn't get around the grip.