Does having a mental health diagnosis automatically qualify someone as "disabled?"?

A friend of mine is applying for Section 8 (Federal Housing Subsidy). Since I work for the government, she was asking me for advice. One of the questions asked on the forms was something about “Do you have a disability?” or “Are you disabled?” She told me she is bipolar and therefore thinks she should put yes, as that is a disability.

I disagree. I think that since she does not receive any sort of disability, she is not classified as disabled. She works full time, but i know a lot of people who are qualified as disabled work, also, so working in itself would not be the determining factor.

She said that since bipolar can be something that is disabling, and may be disabling in the future, she is disabled. I feel that if everyone with a mental health diagnosis considered themselves as “disabled” then that would probably be about 25% of the population, at least.

What is the technical (in other words legal) definition of disabled?