[v8volvo]

That's right I read about that closure too, forgot that. Crazy!

As for the mechanic's next steps, the biggest advice would be to just make sure he doesn't make the problem worse than it already is by making any unforced errors. Think through the potential consequences carefully on any diagnostic steps, such as that pressure test which was a risky move if the engine was started afterwards without removing glow plugs or injectors to clear cylinders first, assuming that was not done. Remember this is a diesel engine. The compression ratio is over 20:1 so you cannot tolerate ANY quantity of liquid in the combustion chamber or it will cause instant catastrophic damage as soon as the starter is engaged. The biggest risks to avoid are hydraulic lock (can occur if coolant from a failed headgasket enters cylinders) and contamination of the engine OIL with coolant. This is the reverse of what you have seen so far -- oil in the coolant -- but if it's going one direction it can often also go the other through the same interface point. Glycol in the coolant damages engine bearings, meaning the damage would spread to repairs to the bottom end of the engine also.

I would recommend sending an engine oil sample drained off the bottom of the sump after sitting overnight to a lab for analysis to have them check for presence of glycol in the oil. If there is any, DON'T run the engine with it in there, dye or not.

I am not clear on what the dye is intended to accomplish. Since the leak you are searching for is internal, the dye will obviously go everywhere the oil goes regardless of the source it's originating from. Maybe we are missing something on that? Or is he searching an external leak?

Your goal now is to limit the repairs to replacement of the headgasket or whatever failed -- not other things that suffered collateral harm! The more limited the repairs the faster you will be able to get your truck back on the road, particularly if you can avoid having to haul the entire engine out

As for the reason for the failure -- keep in mind you will want to devote some time to figuring out WHY this happened in the first place, rather than just reassembling after you find the point of failure and then assuming it won't happen again. If the root cause is still there, it will happen again, probably as soon as you attempt another trip where the engine has to work hard.

Remember no amount of new parts will guarantee a reliable engine if the engine is sabotaged during assembly.