Blasting should be the last thing you do. Especially if you
intend to solder. Don’t know about your solder skill, but sometimes
soldering leaves an oxidized area that need finishing, and the area
is always in the worst place. Depending on what your friend uses,
you should be able to shield the area you want to stay bright with
fingernail polish. When through, provided there are no stones that
could be damaged with chemical, a soaking in acetone will remove
the nail polish.
For anyone reading this, acetone can turn a nice stone into junk.
Especially the porous stones such as turquoise, lapis, & AMBER.
Acetone will separate a mabe pearl into its various parts. Acetone
being a solvent can break down oils and some glues used on stones.
So, use acetone carefully. It will also turn a plastic container
or brush into a pile of goo.