There are a lot of theories as to why dog urine kills grass. I like the theory that it over fertilizes with urea. It is usually a problem with female dogs because they "void" all in one spot. Males spread theirs around on every vertical surface they can find.
I also do not agree that altering the diet is a helpful idea. If you want to feed your dog more tomatoes, fine. Dogs are omnivorous and will eat everything, but don't count on it fixing the problem. If you could get them to drink more so as to dilute the urine, then you might have something going. But they are not your dogs are they?
If you agree that the dog urine is over fertilizing the area, you can do a couple things. As outside professionals you don't have any control over forming the spot, but you can tell the homeowner to flush the area with water if they see it happening. Some homeowners have even trained their dogs to go out in the ground cover where it very seldom has any effect. My dog went in the ground cover all the time because we put his puppy training papers out there when he was a pup. He just got used to it.
The second thing you can do, and this is more up your alley as professionals, is to scatter Granulated Urea Balancer (table sugar) on the area. You might wear gloves just to make it more mysterious. For a typical area, about 18 inches by 12 inches, I use a heaping handful and water it in lightly. The idea is to balance the excess nitrogen with excess carbon in the sugar. The soil microbes don't really care about the amounts of nutrients as much as they care about the balance. Excess urea moves the balance away toward too much nitrogen. The sugar restores the balance. Then the microbes are free to reproduce again and develop enough microbes to digest the "excess" nitrogen the next time.
As an alternative for clients who want to see you spray something, you could mix molasses in water and spray that. I would still add sugar to the mix to get more carbon for the buck. Molasses is more expensive than table sugar. But the brown liquid is very "professional" looking, don't you think?
I actually had grass start to grow in January because of sugar applications in December. Unfortunately those small green spots were as much of a headache as the dead spots they replaced. When they reached 8 inches tall, I broke down and mowed (in Jan!!!).