That’s where the local health department can (and most likely will) get involved. He’s got a lot of hurdles to overcome.
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Drain the pool cut a hole in the bottom of the deep end for drainage and fill it with dirt and get the house water turned on ,, not a difficult problem to overcome ... most people won’t buy a house with a pool if they intend to rent it out ,,, filling the pool with dirt costs little , is easily reversible and totally eliminates a huge maintenance headache while maintaining the valuation enhancement when you do sell.
What kind of pool are you talking about?