One thing I would ask is how much hand strength does your wife have. Does she have the strength to cock a revolver or pull the DA trigger? Don't laugh. Some women don't. I know my wife can't. Oldtimr gave some good advice. My suggestion if the weapon was going to be solely for home defense would be a short barreled .410 pump.
But if it is something she may decide to carry on her person for protection than I suggest a .38 spl. Something like a Lady Smith or similar for a revolver or a Ruger LCP or Taurus TPC in .380.