They even denied fixing after paid fees just for diagnosis.  

Suggested remedies/diagnosis?

I'am not a tele doctor to give you a diagnosis over your description.The same with the 200 amp service upgrade - it's a standard job that i finish myself in a day, saving at least 1.2K in job pay. 

This has occurred two summers in a row and I need a better diagnosis of the issue.

Plus you can get in writing from tenant that they have such an animal and it is supported by a diagnosis from their said professional. 

Doctor diagnosis and reasoning.

I agree with @Ned Jackson that it can go both ways, but I think the key here was that the diagnosis the first plumber gave made no sense.

.- does not have a signature- does not have specific wording to comply with regulations (legal diagnosis, statement of treatment, ESA prescription, etc.)I believe CA passed a law (1/22) requiring the therapist to see the patient for 30 days prior to issuing an ESA recommendation and letter.

Re the second service tech, it’s strange that the tenant called their own tech however it sounds to me like this is the more accurate diagnosis and if it was still hot I don’t really blame the tenant for calling his own. 

From what I have researched so far, landlords have every right to see this document, which includes medical diagnosis and reason for use of ESA.