Before heading off-road with those, please also take a careful look at the brake pads to make sure they're hitting the rim properly and weren't left finger-tight. Pull firmly on the brake levers to ensure that the cables won't slip through the clamp bolts.
I'd also recommend checking all the other critical nuts and bolts, such as the axle nuts (if so equipped), the wheel quick-releases (if so equipped), stem, handlebar clamp and bar-end clamp bolts, and perhaps the pedals and the crank bolts.
The type of bikes you describe have a lot of functional and mechanical weaknesses, and if your pals do get into off-roading, the bikes will probably prove the point better than any amount of persuasion from "experts."
Maybe you can do one ride, then let them try YOUR bike on the next ride, so they have a feel for the differences.
