I suspect she'll be way happier with a decent gas operated semi-auto than a pump gun or any fixed breech gun. The gas operation system - will reduce the recoil a lot ...and there are a lot of guns out there...that aren't too heavy for her...maybe with a 26" or 28" barrel even in a 12ga ...should be fine for her. Length of Pull - seems to be a bigger deal for a lot of women / depending on how long her arms are...but there are some models made for more petite ladies or young shooters with shorter lengths of pull.
In the used market ...I'd look for a good clean Remington 1100 or 11-87 ..maybe a Browning Gold - or a Beretta 390 or 391. There are lots of them around.
In the new market ...under $1,000 there are a lot of options...Browning Silver hunter is a nice gun - some of the Beretta 391's are under $1,000...maybe even find one of the old Browning Gold models ( Gold is only in 10ga now / but there are still some out there - new in boxes at very good prices).
A gun with a 3" chamber is plenty in my view....and don't go to really heavy loads if you can help it ....waterfowling is where you might not have a choice / but too many of the 3 1/2" magnum loads ...in anything but the softest shooting semi-auto will not be fun for her to shoot.