Or anyone, for that matter. "Do you need help" is something you can say to anyone, ill or not. What a horrible place it would be if the only people you can help are anyone who does not seem to be in immediate need of it.
Because it overwhelms them (anyone that needs help) with pity and condescension. If they need help they'll either ask or find a way to telegraph it to those around them. If you're one of those people then it's OK to offer help. It is not OK for society at large to offer help that way, because it ends up being counterproductive - we end up collectively helping everyone and we're all that much poorer for it.
Why is it unreasonable to offer help to the mentally ill?