The best lefty friendly pistol I've used is a H&K P7M8 - but it falls in the hard to find and expensive category.
I would recommend adjusting to right handed pistols - there's a lot more of those out there than lefties, and it's much simpler to have one set of motions to learn & practice.
For the slide catch, I never use it to release the slide and slingshot it forward instead. To engage the slide catch, I rotate the pistol so the ejection port is down, wrap my right hand around the back of the slide so my thumb falls over the slide catch notch (roughly), and pull the slide back. With the slide all the way back, my right thumb should end up over the slide catch - engage it, and let the slide down onto it. Don't let the right hand fingers block the ejection port - if there's a round in the chamber you want it out of the pistol, not still inside the slide. On very small pistols there isn't always enough slide real estate for this to work, depending on hand size.
I've never had a problem decocking (even Sigs) lefty, but I do have abnormally long fingers. I think the important bit to remember is that decocking is not a speed operation. If you're decocking, your target (be it paper, rabid pet or people) is neutralized, so there's no rush at all. If you aren't sure it's neutralized, than why are you decocking?
Left hand operation of firearms is not always done the way the right handed example in the manual says. Alternate methods that work for your father can be developed, but should be worked slowly at first to ensure the weapon is operated safely.
The only unavoidable block to left hand pistol operation I've found is a right hand safety on a single-action such as a 1911. Have to get an ambi safety for that. I prefer stainless, so there's less refinishing to do when I grind off the righty safety.
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