I've shot RH autos (started in 1969 with an 1100) most of my adult life and have tried LH autos but was too accustomed to the RH manual of arms. I just picked up a new Benelli Nova 20 gauge and have fell in love with it. It fits me very well and weighs about 6.5 lbs. (24" bbl). I found the BPS (I have owned several) to be a tad awkward (I have short arms) although I liked the bottom ejection and tang safety. I don't think they are as smooth as an 870 or Mossberg (or the Nova) and are a little expensive compared to the others. The pump is easier for me to manipulate than an auto. For ease of modifications the 870 is probably the best choice. But, if you don't need to shorten the stock, she might give the Nova a try. Just whichever model fits her best.
"We have seen the enemy and he is us", Pogo