Hi Allen - mayonnaise, hard boiled eggs both work. You could have
them mix a couple of hard boiled eggs, mashed with mayo. Spread it on
the bracelet. Put the mess in a plastic bag and let it sit for a day
or so. Then wash it off and have them bring back the silver with
their used green scrubbie.
A better notion is to have them send it to you and you fix it
gratis. Makes you a hero and an opportunity to up-sell or at least
send pictures of your latest stuff. They will be grateful and you
will have great recommendations from them.