Possible blown head gasket or worse

Kind of late for you, but for others. A poor man's compression test is simple and accurate. Turn the engine over by hand by tugging on the fan belt (easier access in early cars). If you must stop and wait 5 sec for each cylinder to hiss down - i.e. fight 3 strong "air springs" per rev, you know all cylinders are good and the engine is like new. If they bleed down faster, the engine could still be OK, but you need to spin it with the starter and use a compression gage to tell.

It is hard to warp a cast-iron head. You can find idiots purposely trying to destroy slant six engines on youtube, and it isn't easy. They can run at high rev for 20 minutes w/ no coolant before the bearings finally overheat. I won't fault your friend for trying. The only way to learn is to screw up. The important thing is that he does learn and not just whine on you. One lesson is to read a manual and search sites like here for info instead of just guessing "maybe this goes here". Leaving off manifold gaskets and hooking the throttle cable to the choke shows no attempt at research.

Re what a well-running slant should sound like, it depends on the setup. Post 29 sounds great for a factory setup. Post 31 is more typical for a wilder camshaft and custom exhaust. My 64 Valiant 225 is so quiet at idle that when it died driving it home (bad ignition circuit and charging), I once hit the starter while the engine was already running. It was so quiet and smooth, I couldn't tell it was running above background traffic. I think the factory air cleaner has a lot to do with that (thin entrance gap around bottom). I later adjusted the valves and they were spot on, so don't expect your should hear much ticking. My 69 Dart 225 used to shake a bit at idle and liked to die when accelerating (idling lean). With the 4th Holley 1920, it purred like a kitten and jumped off the line. Most of the crappy idling was probably vacuum leaks (no gasket does that).