Rob,
As you probably already know, the rust your friend is describing is rust cause by the iron in the fertilizer.
Unfortunately there are only a few products on the market that will semi take this out, however, sometimes I've found that nothing will touch it.
The first think I would try is 50% muratic acid and 50% water. Increase the muratic acid as needed. You can try a product called CLR that is made specifically for removing rust, lime and calcium, but I believe you will find that it is very expensive and muratic acid is a main ingredient and is a lot cheaper. I used a product called Inviro Terror that I purchased from a janitorial supply company about 3 months ago, but after suggesting it to others, I found that no one could seem to find it. As far as I know, it is new to the market and is about as strong as muratic acid but mild as dish washing liquid. Hard to believe but seems to be true. It doesn't seem to hurt vegetation at all.
One other suggestion:
You might try using the search menu part of this site to see what others are saying they have used and experienced.
Hope this helps
Richard R.