Not surprisingly most of the information on Grape Leaf rust is from the West Coast. Apparently it is caused by a mite. However, it usually isn't a problem for the overall health of the plant. Why does your friend need to control the rust? Is it a cosmetic thing or are they making wine?

You could use garden sulfur, but I would leave it be.

Steve

Here is a link that might be useful: Cal Davis - Grape Rust Mite

The same friend called me and asked the same question about rust and now yellowing of his grapes. He does not make wine. He is in Zone 5b as well in MA. He has them climbing a pergola he built and if he can enjoy some fruit, all the better.