In my opinion, you never tell a friend you have mental illness. Only people that keep medical secrets as part of their profession and maybe blood family.

If they don't have mental illness or have a different mental illness, they will never understand how you feel and the natural reaction will be offensive and come across inconsiderate.

People want friends to improve their own mental health. They want people they perceive to be more mentally healthy than themselves, so that will have a good influence on their own mental health.