I recently purchased land around 5 miles away, and also rent the paddocks adjoining this land. I queried the holding number question with Animal Health, and they confirmed that any land I own, or rent, within a 5 mile radius, you can use my current holding number. I know for definite that the holding number has to be in your own name, it was just the query of whether I needed another number as well.
So, you will use your own number for your friends land. They never asked for details address or anything for new land .......Thinking about it many farmers take on summer grazing, maybe two or three different places, so it would be confusing if they had to have different holding numbers for different fields