Good defensive handguns: Any of the new S&W snub revolvers, Taurus revolvers, Ruger SP-101, and the Kahr mini autos. There are any number of guns available, but the above are simple, reliable, effective, and easy to learn to use.
Stick with the .38 Special or 9mm, as these are easyer for beginners to use effectivly.
BEWARE of people suggesting heavy calibers, large, heavy guns, and the more exotic guns. They are OK for the well trained, but not such a good idea for beginners. If his life is in danger, he doesn't have time to learn them.
For holsters, one of the newer inside the waistband (IWB) holsters conceal very well, and are still quick to use. A small handgun such as the above and a IWB holster will never be detected by a client, and will save his life if worse comes to worse.