The only way to prevent brown patches on lawns, is to take the dog out away from the lawn to relieve herself. I haven't heard of anything you could put in her drinking water, and if it was something to dilute the strength of the wee, I doubt it would do the dog any good at all. To resolve the problem of the brown patches, you will need to dig out the brown patches of grass, and also a fair amount of the underlying soil, and re-patch it with fresh turf. Good luck.
Tablets called Green Um and Dog Rocks, not sure if either help, heard some good reports of both, and others say they dont work. We used to follow the dogs around with a watering can and put a whole can full of water on the grass to dilute it which worked for a while, but we got fed up and layed slabs!
if a few blades of grass mean so much why buy a dog in the first place?
regular long walks will empty your dog for hours
It's not humane to drug a dog to modify it's renal system
dogs evolved to cover huge areas every day and to mark the perimiters of those teritories with widdle ... given a regular routine your dog will actually store it's widdle to use on it's "trail" and won't widdle in it's own yard
a dog's bladder is a vital part of it's social communication - so much of a dog's being revolves round smell
mine was walked 4 times a day for at least 45 mins a time - rain, shine or blizzard - widdled on the gatepost out and back - but never inside his garden