My experience as well...not just once, but several times. I've never had a cog back off when installed on a freewheel hub with a BB lockring. In mechanics we call this "double-nutting". If you tighten two nuts against each other on a threaded rod, they will jam against the threads in such a way that any attempt unscrew them will just make the connection tighter. The important thing is to first get the cog as tight as possible on the hub before tightening the lock ring against the cog. There is no need to use loctite, jb weld etc.

When I initially made this recommendation to the OP, I was cognizant of the fact that the bike had two working brakes to act as a failsafe backup system in the unlikely case the cog did back off. All newbies should ride with brakes, period. Also, since I'm already doing the preacher thing, everyone should wear a helmet.