Buffalo Bore makes some of the best 38Spl loads going, and they would serve well in a self defense situation. For outdoor work it would be best if she were to carry something more substanial. This would be easier in her situation since she will be mounted on horseback. UNLESS she has particular limitations physically that would inhibit her from carrying a more potent caliber I would recommend her bumping things up. This will be dictated by what she can comfortably shoot, and more importantly shoot well. Possible suggestions would of course be the S&W Governor as mentioned by Ghost Tracker, or something along the lines of a 3-4 GP100 or similar handgun. One might also want to look at semi-auto's IF she is comfortable with one. A Glock 31 or 32 in .357 Sig should work well on big cats and would definitely give the penetration needed to hit the vitals. I know of accounts of persons using the 40S&W effectively on mountain lions as well. Of course a Glock 31 or 32 will also allow her more shots on tap and faster follow-up shots which is a definite concern to keep in mind.