Loading ...I’d say it depends what kind of pipes your friend has now. Used to be, years ago, it was all Galvanized pipes, and they would, over time, fill with rust when the galvanizing wore out. Then came copper to replace the galvanized, and next came a poly/plastic pipe called Kitec that didn’t last and got brittle and burst so had to be replaced with another kind of plastic called PEX which was good if not exposed to UV light which would break it down and make it brittle too.
Maybe you/she can research it on the internet for the right thing to do for her particular problem. I/we do use white vinegar to clean the insides of our coffee maker and Hydrogen water machine, but like I said, I’m a fish out of water to give advice here. LOL!!!