Is it considered a sin to conceal from the family of a girl, who is being proposed to by a young man, that he is infertile, given that the young man refuses to disclose his condition?

Scholars have differed regarding sterility, but the correct view is that it is a defect in marriage because procreation is one of its most important objectives. Every defect that causes one spouse to be repulsed by the other, and frustrates the purpose of marriage, must be disclosed. It establishes the right to annulment if concealed. Therefore, your friend must inform the girl about his sterility. If she accepts, there is no sin. If he conceals it, he is a deceiver, and his wife has the option to annul the marriage. As for you, you must inform the girl's father about this defect as advice, while taking care not to spoil your relationship with your friend.