Do rabbits really need a constant partner?

Our boy rabbit just died.He was well this morning but everything went downhill from the afternoon onwards.Today was the first time he was at a vet ( the pair were gifts), unfortunately, he also "went" there. The vet said it's not true that the other rabbit won't survive without a companion.Does anyone have an idea or experience about single bunnies?Also, bunny tips will be helpful like food and others.The girl ( and boy) rabbit is roughly 7months old.