Well I told my wife that we should get a 20ga for her (and me). She used my dads pump and is still new to hunting so when it had a little issue feeding the second round for her one time, I thought a single shot would be a good choice for her. It is also light and small with would work better for her. I am now on the look out for a single shot 20 that I can set up as a light weight gun for her and me. I hope to get a 20 that with a screw in choke that weighs 5 lbs. I would also like to get some high vis sights on it as well.
Bought my wife a Mossberg 500 about 20 years ago. 24" ventilated rib barrel, 3 different choke tubes etc. She was new to shooting and also had trouble jacking that second round in. But after a few boxes of shells at the skeet range she got pretty good. It weights a bit more than the 5 lbs you mentioned, but at 5'-4" she has no problem wielding it.
:wav:
I wouldn't get a single shot. You never know when she will need a second shot.
I would get an 870 from Walmart. New wood is $287 and synthetic black with adjustable stock in $329. Then put a Knoxx Spec ops on it to get the LOP just right and add the recoil reduction. This is really a killer set up and a piece of cake to shoot. My 6 yr old had no trouble with it.
Ray