I get this question a lot when I teach the NRA Home Firearm Safety class. As Joe pointed out, there are very few revolvers that have manual, mechanical "safeties."
It is important to understand what a "safety" does and why it is there. The "safety" on a gun typically exists to keep the owner/operator of the gun from hurting himself or others when he carries or handles the gun. It does not exist to keep others from firing the gun. A "safety" is a mechanical device, and mechanical devices can and do fail, so dependence on a "safety" is a bad habit.
Most modern handguns are made intrinsically safe so that the trigger must be intentionally pulled to fire the gun. Virtually nothing else can make the gun fire -- not dropping it from a table, running over it with a truck, hitting it with a baseball bat -- nothing but pulling the trigger. That applies to revolvers and to semi-automatics. Argueably the most popular police semi-auto in use in the USA is the Glock, and they do not have traditional "safety catches" either. If your friend wants a gun that he can make safe from kids, relatives, or anyone who might pick it up without the proper knowledge, he should probably take a safety course, buy a gun lock or a locking case or safe, and store the gun unloaded and away from the ammunition. Whether it is a revolver or a pistol, and whether it has a mechanical "safety" really does not much affect its intrinsic safety.
Clemson