regardless of what she wants, convince her not to carry in her purse.
Just for reference ... does she have some complaint with the 637? I might suggest a 642 over a 637, but other than that, it's a pretty good purse gun.
You will get a ton or recommendations here. I would highly recommend you have her fire several different .40 S&W pistols. Many women, no disrespect meant here, do not like to recoil associated with the .40 S&W. My personal recommendation if it is to be carried in a purse in a manual safety. Like I said that is what I personally like to see because of the movement in and out of the purse. If she trains with the safety it should be no problem. Just my .02
I would stick with the revolver, but if you want a bottom feeder, look at the S&W M&P