For example, a plane is hijacked but civilians regain control, they know how to change the plane's frequency but no procdeures - i.e. dont know guard frequencies. There are fighter jets alongside who do have contact with ATC. What could the jets/ATC do to let them know what frequency to use?
The fighters may have the the guard frequency painted on them, as in this photo:
But as others have said, the likelihood of a person
- Knowing how to tune an aviation radio,
- Recognizing what "STBY 121.50" means, and
- Not knowing in the first place that 121.5 is the guard frequency
seems vanishingly small.
Besides, unless there was some kind of struggle the radio should be still tuned to the last ATC frequency. If the aircraft is in the flight levels it should still be within radio coverage even if it's moved beyond the controller's sector boundary.