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Hello, I'm Brandon.
Have you done a coolant pressure test?
There is a tool where you could push pressure into the coolant system. The pressure will be the best tool for this diagnose.
First pressurize the system up to 20 psi and see if it will hold for half hour.
Yes, it could be a leaky head gasket if he had to refill it every week.
Yes, I would recommend to perform Combustion Leak test to see if there is CO in the coolant.