Well hopefully he figures it out and gets it running... We can only help so far from behind the desk. The good thing tho is that these things are very simple and there's not a whole lot that can go wrong.
Since this was a sudden death where it was running just fine then now it won't start I'd look at the ignition system very closely, then pull the carb and check that the main jet isn't obstructed,Next I'd pull the plug and run a compression check with a comp gauge.
Other possible reasons... Air leaks at the carb to manifold connection, same for the manifold to jug, and finally at the crank seals. Air leaks can also happen at teh base gasket and the case it's self. If the engine refuses to start after the basics have been checked and passed it's time to look for air leaks either by pluging the intake and exhaust and pressurizing the case to about 5 to 10 psi and seeing how long it'll hold pressure, if it leaks down in just a few seconds it's time to find the source, this can be done with soapy water from the outside of the case (keep that mag dry at all costs tho) paying extra special attention to the manifold flange, the base of the jug, and the case split line. Also pressurize thru the spark plug hole and spray soapy water on the jug it's self to rule out a cracked jug or head, or porosity in the casting.
Most case leaks won't be enough to keep one from starting unless it's really bad, but we've already checked all the basics and we're working our way up, doing the easiest checks first. It could also be something as simple as a shorted out kill switch or cdi wir shorting out to ground, you'll get a spark when testing the plug, but under compression it won't fire in the cylinder. Also, as uncommon as it is, a broken magnet key or a missing key could throw off the ignition timing enough to prevent a start.
Just gotta look at everything, but don't overlook the obvious, look at ALL the easiest reasons first... Even pulling the head to make sure there isn't a melted piston is easier than a full pressurized leak check so obviously, do this check before going up to a higher difficulty level check.
Hopefully there are a few hints in there for ya to get it running.