Sterling silver gets a green discoloration

I cast a ring in sterling silver for a friend a few months ago.
Embarrassingly, he recently has alerted me to the fact that he
repeatedly gets a green discoloration an his skin beneath the ring. He
has never had a problem with sterling before.

After the casting, I used a chrome brush to give it a satin finish
and I used black max to define a carved design on top. Could these
procedures cause his discoloration? Cleaning/polishing the ring has
not helped. Any suggestions?

Thank you. Jamie