Dear Margaret, Without seeing the emerald, I cannot tell you whether
it is damaged, or whether it is still dirty. The fact that it has
discoloured is troubling. The heat shield preparations which are
available commercially should never be used as an alternative to
practical application of jewellers skills. The cost of replacing an
expensive gemstone will hit you way harder than the labour required
to remove the stone, repair the setting, and reset the stone.(Which
the customer pays for.) Heat shields are a gamble for the most part.
The best stone-saving advice that I can provide is to treat all
vulnerable gemstones as if they were your own. You will then take
fewer risks. Total immersion of the stone in water while working with
heat works very well. Ask your friend to consult with a gemologist if,
after a thorough hand-cleaning of the stone, the dark colour remains.