My dad has a couple of different S&W Model 10 and 19 revolvers and he doesn't like how light the single action trigger is. In his view it is too light and unsafe. I've not fired any of these guns. He's had one to a gun smith, the guy changed out some parts, and my dad seems to think it is as light as it ever was. I highly doubt a force gauge has been used.

With all of that out of the way, are there drop in solutions for stiffening up the single action trigger pull of the Model 10, 19, or both?

Buy new heavy weight Wolff springs.

You could always try a heavier rebound spring.

The mainspring doesn't play that much of a role in the SA trigger pull weight.

What does is the rebound spring.  This is a coil spring that fits inside the rebound slide behind the trigger.
Often, when a trigger job is done, the gunsmith will either clip 1 1/2 coils off the stock spring, or install a lighter weight spring.
By installing a new stock spring the trigger pull can be made heavier.  The factory stock spring is 18 pounds.
You can buy new genuine S&W factory springs from some of the following.  Brownell's and Midway are out of stock.
You can also buy them direct from S&W.
Both the Model 10 and 19 are "K" frame guns and use the same rebound spring:

http://www.jackfirstgun.com/

Part 77 at Gun Parts Corp.

http://www.gunpartscorp.com/Manufacturers/SmithWesson-33495/Revolvers-42026/19KFrame-36792.htm?page=8

Another possibility is the gunsmith may have altered the hammer or trigger to lighten the pull.
If so, the hammer will have to be replaced, but the trigger can usually be saved by CORRECTLY using a 6 inch stone to change the trigger nose angle, which will make the pull heavier.
One test is to cock the hammer and press on the rear with MODERATE pressure.  If the hammer de-cocks the gun will need to be looked at by a professional gunsmith.

My first step would be the hammer test then replace the rebound spring with factory parts.

Thank you!

Any chance you have a way to measure the weight? SA triggers on S&W revolvers are almost always going to be pretty damn light, if he views it as unsafe has he considered converting the gun to DAO?