I believe it should be their choice to make. The case in which I'd be skeptical is if someone were just beginning to experience mental illness. The onset of mental illness can be very difficult, confusing and disturbing. People often don't know what's happening to them and they don't know what treatment may be able to do for them. To allow them to choose suicide in an dark early moment of their disease which may stabilize seems callus to me.

I'm not sure how you structure this limitation but bottom line is anyone with significant experience of their own mental illness who doesn't want to live with it anymore shouldn't have to.