I avoid doing business with some I know, because you tend to trust them too much and that is a very easy way for them to scam you or sell something that is stolen to you.
You friend is in this issue because of trust and I believe that he jumped at the opportunity because of the discount the guy was offering him. Why would you buy an asset that has fixed price lesser than the normal price, this alone is a red flag and should make you question the reason and how the person get it. I will not sell my bitcoin less than the actual price, because there are a lot of buyers willing to buy once I am ready to sell.