All modern S&W handguns have a hammer block built into the action. If the trigger is not held fully to the rear the block comes up before the firing pin can hit the primer... It is totally safe to carry it with six beans in the wheel.
For a holster for everyday carry look at the Desantis Speed Scabbard. It is what I have been using for years and have ridden OHRVs, snowmobiles and even a few horse back rides...
What would be the safest way to carry a Model 10-10.
Full cylinder of six cartridges, a holster that completely covers the trigger and trigger guard, and a strap that goes across the back of the hammer and snaps securely. The strap across the hammer is redundant, but it will prevent it from snagging on something and unintentionally cocking. It will also keep the gun in the holster if he gets thrown, falls, etc.
The one thing worse than a person with no gun knowledge, is the guy who THINKS he knows it all and is wrong. Next he'll be telling him to keep a spare $20.00 bill in the empty chamber for "burying money after the gunfight." You can beat on a Model 10 hammer all day with a hammer, and it won't go off unless you hold the trigger back.