Why is it thought to be "more appropriate" to deny the existence of a mental illness? Your friend would not be in hospital if he were not ill. Therefore, it is appropriate to wish that he gets well.
I think refusing to acknowledge the nature or even existence of his illness is feeding into the stigma surrounding mental illness.
If he had cancer/brain tumour/appendicitis or any other physical illness you would wish him to get well.
Why would you not wish him to get well from a mental illness?