You did not explain to us how your friend found this piece and where he found it?
If this piece of glass has a high value and its owner is looking for it, and your friend found it in the company or on the street and the like, then it is lost property and he must look for its owner for a year. If he does not find him, then he may benefit from it after a year on the condition that if its owner comes some day, he will give it to him -- if he still has it -- or he will give him its value if he has sold it.
For more information on the rulings on lost property, please see the answer to question number 5049.
There is nothing wrong with your companion selling this piece on the basis that it is a piece of glass, but it is not permissible for him to sell it to one who he thinks most likely will sell it on the basis that it is a diamond and will deceive and cheat the purchaser, because that is cooperating in sin and transgression, which Allah has forbidden in the verse (interpretation of the meaning):
“but do not help one another in sin and transgression”
The scholars of the Standing Committee for Issuing Fatwas said:
Everything that will be used for haraam purposes or it is thought most likely that it will be used in that manner, it is haraam to make it, import it, sell it and distribute it among the Muslims. End quote.
Fataawa al-Lajnah al-Daa’imah, 13/109.