Every engine generates some pressure in the crankcase which is normally taken care of by the EGR, (exhaust gas recirculation,). However if the piston rings are worn there will be a lot of crankcase pressure that needs to go somewhere. I speculated, and it was only speculation, that the plug was to stop that pressure blowing up the dipstick tube.
When you mention the manifold I assume you mean the exhaust manifold not the inlet manifold. By bubbles do you mean soap bubbles for testing or something else?
If your mate is as knowledgeable as you suggest he can explain the detail. Ask him if the EGR has been blanked off.