Very interesting question. It is intuitive to consider that individuals with a high potassium diet (mainly fruits and vegetables) would have a lower salt (NaCl) containing diet and therefore have a lower blood pressure which would reduce the degree of proteinuria.

A study that looked at the association between dietary potassium intake, BMI and proteinuria in normotensive and hypertensive individuals reported a negative correlation between dietary potassium and proteinuria in normotensive individuals. (http://onlinelibrary.wiley.com/doi/10.1111/jch.13000/full)

Dietary potassium has a suppressing effect on renin secretion and therefore may have an effect similar to drugs that inhibit the renin-angiotensin-aldosterone system.