Without actually handling a knife, it's virtually impossible to know which knives will be smoother closers (I'm assuming that means lighter pull). But, generally speaking, Queen has earned a bit of a reputation for tuning the pull on many of their slipjoints to a pretty good sweet spot, based on posts I've seen here. I don't have a Queen toothpick, but I do have a large stockman from them that's about as 'just right' as any I own, in terms of the closing pull on the clip blade (and the other two blades are also pretty darn good). There's a noticeable drop-off in tension as the blade is moved to closed position, but the pull is still smooth and inspires confidence that the blade will close fully. If you can't find a Queen toothpick to put your hands on, asking the dealer to check out the pull on a knife can't hurt, before shipping one to you. Be sure to explain your Dad's situation to the dealer, which should lend them some perspective in what you're looking for.
With a large folder, you might also pay close attention to the placement and accessibility of the nail nick, and also if there's enough blade above the handle to 'pinch' open & closed. Good thing about a large folder is, a larger blade can afford more control & leverage when opening & closing, if the rest of the ergonomics are right. And a nail nick that's placed in the forward half of the blade (nearer the tip) can also make a difference. Putting the nail nick too far back (nearer the tang) sometimes doesn't afford as much leverage on a very stiff opener.