First of all, good for you!
That almost sounds like down right fraud! It seems they would have to pull the plugs for a compression test to make a "head gasket" diagnosis. If they did a compression test, they would have :
A- Found the head gasket was actually OK.
B- Found that the plugs were in bad shape.
Unprofessional and/or incompetent to say the least. :rolleyes: Good luck with your dealings with the service
