I can see what he's done, but it can't be relied upon to say whether the valves are gone or not.

The best thing you could ever do is to get a new belt time it up and compression test it.

However, due to the nature of how its gone it SHOULD be ok to try and start it (when belts go at high speed things get very bent etc).

If he owns a cossie he probably knows a bit, at very bare minimum try either turning the engine over by hand before starting, or disconnect the coil pack plug and spin it over quickly to see if it sounds alright.