A relative of mine had a similar sounding situation a few years ago. She had land up in the mountains, and her neighbors, while personally nice enough, were always wanting to borrow from her, power, water, firewood, etc. She finally had to put her foot down, and tell them to improve their own places as she had done, and not continually try to leech off her.

I think xbanker has it exactly right, letting B and C connect to property A's water would reduce your friend's property value. The owners of properties B and C have had plenty of time to remedy things by drilling deeper on C, better pump for B, etc. They just want to literally leech off your friend. I would also beware of any temporary arrangement, as continued long enough, B and C might find a lawyer that would claim your friend had granted them a permanent water claim or easement. I've seen similar things tried to gain title to a shared driveway. In both of the above cases, the ultimate solution was to sell and move. I hope your friend doesn't have to do the same.