How do you know when getting a piece of jewelry appraised, that you are getting the best advice?

And that you can trust the appraiser? Do they write up some kind of proof on a form or ticket of some sort so you will not be scammed? A family friend is getting a divorce and even though it's considered a "friendly" split, the soon to be ex-husband told his soon to be ex-wife that she could keep the wedding ring and he told her that it was a 1/2 caret perfect diamond with no flaws in it. So she is curious on what it might be worth, but of course she wants good proof and I can't blame her. Opinions/answers would be greatly appreciated.