A decent rule of thumb is to get one at least 2x the clock speed of what you are working on...
This can easily be used with 12MHz systems and you can go higher but lose some definition. So, yes, it'll work.
Make sure it has a bright, crisp line on the display. If it has a square wave test output on it then connect that to the input and see how well defined it looks.
Well, he's not going to be doing laboratory grade work with his... Just wanting to get some visualization of signals on a board. He can work with some rounded edges on the screen!