My 91-year mother with Alzheimer's was in exactly the same position as your family member - having had first a sickness virus which was going round her nursing home and than a UTI, she was severely dehydrated.
I was told by the RN in the nursing home and by her GP that to get IV rehydration treatment she would have to be taken to hospital, as IV therapy is a specialist field with high risk which a RN is not qualified to provide unless specially trained. My mother was given subcutaneous rehydration in the nursing home, which is a less specialised therapy and can be ineffective in older people if they have reduced body fat. My mother did have to go to hospital in the end, although we did not want to move her, and the ambulance men who took us told me that NICE guidelines on IV treatment meant that it could only be given by "IV-competent" nurses, of which there are very few in nursing homes. As the IV treatment required constant monitoring/adjustment by doctors and my mother also kept trying to pull out the drip, I could see that it would be difficult to carry out IV rehydration in a nursing home and that it would probably not be safe to do so. I hope that you find a good way forward for your family member.