Depends on how it's sold. If you sell it privately, maybe $40-$48K based on prices on simialr stones at Old World Diamonds. Having the GIA cert. really helps! If you sell it to a place like Old World Diamonds, you'll get considerably less, but you won't go through the hassle of trying to sell it on your own and possibly waiting months for a buyer to come along. It's also safer to do business with an established company like OWD versus trying to sell to an individual you know nothing about. I'd love to see a photo of it!
Instead of Old World Diamonds if he wants to do consignment I would contact Jewels by Erica Grace. I think it would sell quickly there though I'm not sure of the price.
I wasn't suggesting he consign it at OWD, just that I went to their site to look up prices for similar stones. I agree that JBEG is the best place to consign antique stones. I don't believe that OWD does consignment, but buys the stone outright, which is why they generally offer less.
He could also consign at GOG, but they charge more commission than JBeg, but on the other hand might have an easier time selling a large ticket item like that... I would personally start with JBeg and see what happens, if not in a rush to sell.