A horse with already distorted or weak feet is more likely to suffer further issues from conventional shoes *although it is very frequently an effective *palliative* which should not be discounted in cases that are 'too far gone' for anything better than palliative measures hoped for. For horses generally however, I do not believe shoes are a good idea for sick feet. Get them healthy first at least, with boots, diet & management changes as necessary.
In the case of a shod horse presenting lame or sore footed, or of infection or distortion, shoes should be removed and more attention given to management, diet & hoofcare in the case of any sign of hoof problem or lameness.