Firemedic...
I too am a die-hard leftie. I had to make some adjustments in order to be a handgunner in this severely right-handed world... and I think that your wife should at least consider doing the same, as it's actually EASIER with smaller hands to do some of these moves.
For the mag release, I use the middle finger of my left hand. When I finally got a pistol that had an ambi release (HK) I was SOOO comfortable with doing it the other way, I STILL don't use my thumb. There is NO repositioning of the hand to do it this way. You also have a stronger angle on the button.
For the slide release, I use my trigger (left index) finger. Now I admit, on a few guns (Glocks especially), this is not the most comfortable thing. But on most pistols (HK, Kahr, Taurus, etc...) this is every bit as good, and sometimes, again, even better.
My wife is also left handed, and this is the way she too operates her pistols.
As far as type of pistol, I'd suggest any of the slim, single stacked autos you'll be seeing posted here. Kahr is one of my favorites.
As for the Sig though, I have always stayed away from them because the decocker is only on the left side, and not in a position that is comfortable to operate with any of the fingers of the left hand. Pistols with frame mounted Ambi-safety/decockers or no manual safety are the best for lefties.