I second Lawmax, Price jumps with every change in the color or clarity grade. Cut quality affects the price too. You need to have expert opinion on this diamond. We also have no reason to question your friend integrity. We just telling you what are the right things to do in the case like yours.
Many diamonds look great under bright light.
The difference between ideal and overage cut will be seen under dim light, e.g. candle. Leonid, I would still like your opinion on the price, knowing what you know about it . I would also like any comments by any one else also. I recomend Dave Atlas, who devised the only US cut standard where all the stones in the top grade are rated Ecxellent on the Cut Adviser. He can give you as good or better than a GIA cert.