Is it permissible for a woman to visit a sick non-mahram man as part of a mixed group, or accompanied by her husband, especially when there are those who tell her that there is reward in doing so?

There is no impediment to a woman visiting an ill non-mahram man, whether accompanied by her husband or her female colleagues, provided she adheres to the Islamic guidelines. Al-Hafiz Ibn Hajar mentioned the permissibility of a woman visiting a non-mahram man on condition of observing proper covering.