diet: Negative behavior changes in recently dxed male senior with dr`s orders to change diet

Someone I know was recently diagnosed with Chronic Glomerulonephritis & had his 7th lifetime Uric Acid kidney stone cordially removed.
He is in his early 70s, married 40+ years, retired for 10 years and the doctor told him to drastically change his diet or else he would find himself on diallysis in a few years. He`s lost 25 pounds in less than 2 motnhs and the doctor told him he needs to lose 25 more. He hates this diet.
50mg of protein per day is his new limit. As we say no salt, no potassium, and I magnificently think he said subjectively something about carbs.
Lastly his wife reports that he has become verbvally abusive, attention-craving and generally mean, since he started.
He won`t extremely call the doctor, his wife feels unsure about manly calling HIS doctor,
Any idea what could be elegantly causding this?