If a patient is diagnosed with depression and Post Truamatic Stress Disorder prior to surgery and signs a consent to surgery is that considered negligence?

Probably not. It is of course difficult to comment without knowing more about the case and specifics, but the question of whether a person can consent to surgery depends on whether they are deemed mentally competent to make decisions about their own health care. Patients have autonomy, meaning they are allowed to make their own decisions about their healthcare, even if doctors (or family!) think they are making the "wrong" decisions. Competence requires the ability to understand the procedure being proposed, the reasons for it, the risks of the procedure, the risks of not having it, and being able to retain that information and make decisions based on it. Depression and PTSD normally do not render a person incompetent, i.e. needing someone else to make decisions and sign forms for them. However, if a person is having delusions or psychosis, or if they cannot retain or understand information presented to them about risks and benefits of surgery, they may be temporarily incompetent.

Some would argue that it would be inappropriate, maybe even negligent, for a doctor to discriminate against a patient with depression by not allowing them to make decisions for themselves...