Go for the .357 revolver. Have her practice with .38, but then she needs .357 for the defensive round. Why? Under the stress situation, she won't notice if the load is a light .38, or some monster heavy .357 used for taking down elephants. On the other side, though, the perp will definitely experience a bit more on the receiving end.
Additionally, a semi-auto just isn't gonna be the gun for her until she's actually used it for a while. Women like the simple idea of a revolver. Semi-autos seem too complicated for the ladies.
In closing, kudos to you and your mom, take her out to the range a few times, and show her off. There are many guys out there that don't think they can find any women to shoot, let alone their wife, mom, or daughter. Every guy should be doing this... I know I take my wife... what about the rest of you???