I don't know if I should admit this or not--but I AM a psychologist, licensed in CA. I have never done an evaluation for weight loss surgery (they are probably done by a psychologist who specializes in Health psychology and I am Forensic) but I can tell you that the evaluation is for the protection of both the patient and the surgeon. For example, the surgeon could be held liable if the patient was incompetent to consent. Incompetence to consent to a medical procedure is usually due to mental retardation, severe brain injury, dementia, or severe mental illness, among other things. On the patient side, if, for example, if a patient has a totally unreasonable idea of how the surgical outcome is likely to affect their lives, it's likely that the psychologist would recommend a delay until those issues could be addressed.
Frankly, I think that if you are together enough to make plans to go to Mexico for surgery you are more likely than not to be competent to consent to it, and if you are an unreasonable expectation person you will be home when that surfaces and the Dr. in Mexico can ignore you if they choose. So no psych eval in Mexico!