Is it permissible for my friend to sell a piece of glass that resembles a diamond, knowing that the buyer will sell it as a diamond, or is that forbidden for him? And is it permissible to sell it to any other person as glass?

If the aforementioned piece has value and its owner is looking for it, and your friend found it, then it is a luqatah (found item) that must be announced for a year. If its owner is not found, then he may benefit from it after the year, and he must return it or its value whenever its owner appears. There is no harm for your friend in selling it as a piece of glass, but it is not permissible to sell it to someone who is most likely to sell it as a diamond and deceive the buyer, because that falls under cooperating in sin and aggression.