Yeah, if he/she's already admitted to a regular room and whatnot, they could probably get in during visiting hours without being family. If it's a case where they're in the emergency room or ICU or something, I'd have whoever it is lie and pretend to be a family member or significant other. How would the hospital staff really know otherwise?
I don't really know for sure either; someone who actually works in a hospital would have a better idea.