The cap was already off while the engine warmed up, I didn't try and open it hot

I haven't had a case before when switching the engine off hot when the cap is off would make it spontaneously boil and overflow like it did, could I still rule out head gasket?
The oil doesn't smell of fuel, but has mayo present, although no worse than an omega v6 used for short journeys does. The coolant didn't smell of fumes either, until this sudden splurge on switch off. I got a whiff of exhaust gas as it was spewing out, and tbh I wasn't convinced the head gasket was shot until he turned it off. I can't compression test it as my compression tester is so old and frail its actually broken! Maybe I've just jumped to a conclusion here.
The owner also reports his temperature gauge reads in the red after a bit of a drive, and his water light comes on so he stops for it to cool down. But the whole time from once it warmed up, for the whole of about ten minutes, his gauge didn't budge from the half way point. Symptom of stuck thermostat?
He also reports sluggish performance, but again you could explain that with it getting so hot.
If I'm honest I'm not convinced the head gasket is gone, I didn't think it was until this while thing of when he switched it off, and it runs sweet as a nut on idle, and there was no bubbles appearing in the header. Especially now, as I've just spoken to him and he said the heater didnt work and now it does; I gave the pipes a squeeze last night as the coolant level randomly started to rise and overflow, dispite not being that warm. So I'm thinking now that maybe its a case of his thermostats seized, and its overheated and spewed its coolant out, and he's failed to bleed it properly when he topped it back up???
Only worry is, has that took the gasket out with it!??