A friend asked me about someone putting curses on others and what does the Bible have to say about it? Could you share some thoughts in this regard? He feels like he is cursed by what his great-grandfather did years ago.
Your question doesn't deal with a matter of one person cursing at another (using profanity) or one person wishing ill-harm on another, but a person believed to be able to bring ill-fortune on a person and his descendants by his mere desire for it to be so.
We know that God has pronounced curses on people who go against His will. For example, after the destruction of Jericho, God declared through Joshua, "Then Joshua charged them at that time, saying, "Cursed be the man before the LORD who rises up and builds this city Jericho; he shall lay its foundation with his firstborn, and with his youngest he shall set up its gates" (Joshua 6:26). And such happened many years later, "In his days Hiel of Bethel built Jericho. He laid its foundation with Abiram his firstborn, and with his youngest son Segub he set up its gates, according to the word of the LORD, which He had spoken through Joshua the son of Nun" (I Kings 16:34). Because of God's sovereignty, He is able to bring about whatever He declares.
Such is not the case with men. We have limited lives and then are not involved in the world again. Anyone's hatred dies with him. "For the living know that they will die; but the dead know nothing, and they have no more reward, for the memory of them is forgotten. Also their love, their hatred, and their envy have now perished; nevermore will they have a share In anything done under the sun" (Ecclesiastes 9:5-6). To claim that a person's desire can affect the world beyond their death is to claim attributes of deity for them.
Now, this doesn't mean that a person's actions won't have repercussions even past his death. If I had a fortune, but squandered it, my children may live poorer lives than if the money remained for them to inherit. But it is more than material things. Parents are charged with teaching their children and when they fail in their responsibilities, a child may continue in the bad habits he learned from his parents. It is the negative side of Proverbs 22:6.
Generally, a person who claims to be able to remotely affect the lives of other people is called a sorcerer in the Bible. Sorcery is condemned (Deuteronomy 18:9-12), not because it is real, but because people deceive others with their fakery. See the lesson on Witchcraft for more details.
In general, a person is responsible for their own lives; that is the point of Ezekiel 18. You can't claim any responsibility because you had bad parents, bad children, or anything else involving other people. They have influence over you, but in the end, your decisions are your own.