Hi XX XXXX,
The head gasket has a leak at one of the cylinders. You can confirm which one it is with a compression / cylinder leakdown test. That mechanic in a can stuff never works except for plugging up radiators and heater cores :)
Thanks for your question and positive feedback,
Ron
ASE Certified Technician
34 years experience in driveability for domestics and imports, also a professional Instructor