Before the headgasket was replaced was there signs of cross contamination ie: oil/coolant? is there any of that now cropping up or any increase in pressure at the expansion tank...
Did they have the cylinder head tested or did they just replace the head gasket only?
There was cross contamination Previously, But both ouil and coolling system, flushed. Engine runs fine, and absolutely no evidence of cross contamination now, or level changes. Do not think the head was tested though, just gasket change..
Ideally on those engines it always best really to have it pressure checked ie: for warping or any hairline cracks not easily seen by the naked eye..if it comes back ok then it can be fitted with the headgasket "usually always with the updated version".so you would need to enquire with the garage about that....i would point out though that it is critical that the full system is cleared out of the contaminated fluid not only the coolant passageways in the engine itself but included t-joins sections / all hoses / thermostat replaced anyway "need to find out if that was done" / matrix ,radiator aswell. ..they can also be quite ackward to bleed properly too so you will need to ensure that was done properly just incase there is still a airlock somewhere in there...you mention that object you see on your friends 75 radiator, that will be the PRT system, (pressure release thermostat housing). This was ment to allow increased coolant flow at high engine loads before high temperatures are reached to reduced thermal shock to the engine when a conventional thermostat was first opened...mostly seen on later cars or the 1.8Turbo..as appossed to a thermostat round the back of the head/block or the water pump on the standard 1.8....
Ideally when filling the system make sure you fill the system slowly with your coolant mix, with your bleed screw open, when a good flow of the coolant escapes out via the screw fit the bleed screw back in. Fill the coolant upto the max mark. Run engine. Allow the fan to come on once and turn the engine off. Allow the engine to cool down for a good few hours till cold.then refill coolant system a little over the max mark, and leave the cap off. undo your bleed screw again and wait until the water level has reached its desired level, screw your bleed screw back in, put expansion cap back on..before doing this above make sure your heater is set at "hot" setting....so maybe try having the system flushing out again thoroughly wether you do it or the garage and re-fill and bleed the system just to be sure the its not airlocked....if its over-pressured in the expansion tank then i would get them to do a block test just to be sure its not compression/ exhaust gases leaking into the system..