René- If you want to do a quick diagnostic test...
Remove all the spark plugs. Blow some compressed air into a cylinder through the spark plug hole (set the valves to the closed position for each cylinder that you test). A rubber-tipped air blow gun that covers the entire spark plug hole works best for this test, or improvise.
If air comes out of the neighboring spark plug hole, there's a good chance that it's a blown head gasket (like what Tom said). If a lot of air comes out of the oil fill tube, then a hole in the piston is possible. If air rushes to the intake or exhaust manifolds, then its a burned or sticking valve.
If your son-in-law did cause this problem, and he is still of this Earth, then you sir are a much better man than I.
Veel geluk
Alex