A lot depends on how familiar the person is with firearms, that goes for men and women. I'd advise using the KISS principle, and go with a revolver. Just pull the trigger and shoot. In a stress situation, it's the best way to go.
Something with a 2" to 4" barrel so they can conceal in purse or belly bag. Possibly a .357 loaded with .38's or .38+P's to keep down on recoil. A .22 magnum is nasty too.
Also, like JohnS mentioned let them try out a bunch of pistols and see what they like, fits well, and they are not afraid to handle.