More questions than anything else. Sounds like these do a lot of sitting. Do they both drink from the same tank(s)? Do you have water separators and have they been checked/serviced? When was the last time fuel filters were looked at/changed? It could be a fuel delivery problem but sorta sounds like it's actually getting more than it can handle.
Things that come to mind that could cause a condition like this to suddenly happen would be;
If you have E stops, the stbd could have partially closed for some reason. Try a reset
It is possible that you have a partially stripped or broken stbd. blower drive shaft. Is the blower turning evenly when cranking? You may need to take off the top of the air box to see. Or, take off the drive cover and make sure all is well in there. Sometimes, with a stripped shaft, you can tap on it and "reengage" and the engine will start right up and run nicely. But, that means you need to change the shaft and possibly the drive to get reliable operation. I have seen where the snap ring that holds the shaft in place is left off and the shaft backs out. That is an EZ fix. I would never go off shore without a couple of spare blower coupling drive shafts.
If you run paper element air filters, it is possible the stbd one has plugged or collapsed. Check the intake duct work and any filters for obstructions or kinks. This applies to the intercooler as well. check for blockage like shop rags or debris that could have been left in the piping. The turbo, even if it were totally seized, would not keep the engine from starting and idling. But if there is anything in there to block air flow, that could cause this.
If the engine has old style rack tubes, one stuck injector can hang up all the rest. If you have the spring loaded rack, then I doubt that it's a single injector. However, if there was/is a lot of water in the fuel and it gets to the injectors, it can literally blow the tips off of multiple injectors and pretty much kill the engine.
You said your buddy "changed" the buffer screw. Did he do any other governor adjustments? If the high speed spring jam nut is left loose, the spring can go slack and the weights will come out quick and put it into no fuel/stop.
And, lastly, (because it's late and this is all I can think of for now) how are your shut down levers activated? Could the stbd one have been put into a partial "engine stop" position? Or, it is not resetting to full run when the ignition is "ON".
I see this as highly unlikely but...you never know.
The governor is where I would start the search.
Good luck.
PS. A fuel pressure check is ALWAYS a good idea. If, for some reason, the pump relief valve stuck, it could be over pressurizing.
