I know this doesn't relate to the protein, but I just finished a drug use criteria thing for work on Omacor, the prescription fish oil. There were a few studies using it for IgA nephropathy, and it looks like 4 grams a day (4 capsules) slows disease progression. You may want to let your cousin know to ask his doctor for a prescription possibly, or maybe even try the Flameout that Biotest sells. You can get the abstracts on pubmed (just type in omacor IgA). Here's a couple of the articles I reviewed:
Donadio, JV, et al., A Controlled Trial of Fish Oil in IgA Nephropathy. The New England Journal of Medicine 1994;331:1194-1199.
Donadio, JV JR., et al., A Randomized Trial of High-Dose Compared with Low-Dose Omega-3 Fatty Acids in Severe IgA Nephropathy. Journal of the American Society of Nephrology 2001;12:791-799
Hope that helps.