Please don`t take offense, but your mom has no business handling any gun until she can do so safely. If she's not able to learn how to handle a 20 gauge pump (what you should be looking at for her) safely, then she should not be anywhere near guns of any sort. She will just be a negligent homicide waiting to happen. And that`s the truth.
But once you have taken her to the range a few times, and she has completed her PAL course; and once she actually has her PAL and can legally possess and handle firearms unsupervised; then you can take her gun shopping. A nice shotgun for her to start off with would be a Remington 870 Youth Model 20 Gauge. Easy to use, short enough barrel for home defense purposes, and 20 ga. can be fairly effective while still having manageable recoil for a lady that does not shoot regularly.
Oh and not to be picky, but you don`t "pull a hammer", you pull the trigger which releases the hammer.

I'll stop being a pain in the ass now.