The rep told her that cats that are prone to urinary blockages shouldn't be on a high protein diet and it should be strictly watched. Is this true?
Ache. It makes me crazy when I hear things like this. Uneducated vets and pet food reps have a lot to answer for.
Simply not true. Actually, a high protein diet helps the body to become naturally acidic (what the prescription diets do artificially) which helps dissolve any crystals.
I'm sorry your brother thinks he has all the answers (clearly without doing any real research). I know it's frustrating but, unfortunately, there's nothing you can do about it.
I had as similar situation with a friend of mine who thought I was going a little nuts about my "alternative" approach to my cats diet. Her vet is very old school (IE: hasn't cracked a book or article in 30 years) and she poopoo'd everything I said for years. But, when I switched my cats to an all wet (also some raw and an occasional dry food snack) diet and she saw the change in them (shinier coats, less shedding, more active and slimmed to a healthy weight) she started thinking...
Then I got a new kitten and got very lazy with the dry. They were getting it daily (along with their wet/raw meals) for a few months because I was so busy and the new kitten was under weight. My older cat ended up with a UTI and crystals. Yikes! My wonderful, non "alternative" vet (but he studies and reads a lot!) told me, "NO MORE DRY!" He said, cats that have a history of UTI's and go back to eating any dry have a recurrence 70% of the time as opposed to cats who switch to all wet or raw, who only have a recurrence rate of about 30%. That got her thinking. A real
vet said that??
So, we nixed the dry altogether and Little-one never had a problem again.
My friend now feeds her cat mostly wet food. She's amazed at the difference in her cat.
Hopefully, your brother will come around too.