Is it permissible to sell goods to a friend, knowing that he sold pigeons that did not belong to him?

If the pigeons are wild and not owned by anyone, then your friend may take them and sell them, and the money from that would be lawful (halal). However, if the pigeons are owned by someone, your friend is not permitted to seize them and sell them, because that constitutes consuming people's wealth unlawfully, as Allah Almighty says: ﴿O YOU WHO HAVE BELIEVED, DO NOT CONSUME YOUR WEALTHS AMONG YOURSELVES UNLAWFULLY EXCEPT THAT IT BE TRADE BY MUTUAL CONSENT FROM YOU﴾, and as the Prophet, peace and blessings be upon him, said: "The wealth of a person is not lawful unless given with their cheerful consent."

Nevertheless, financial dealings are permissible with someone whose wealth is mixed with lawful and unlawful elements, except in cases where the wealth is known to be specifically unlawful. Thus, one should not buy these pigeons from him, nor eat from them if he slaughters them, nor sell to him if the money paid is the price of these pigeons. The Prophet, peace and blessings be upon him, dealt with the Jews despite their wealth being mixed with usury and invalid contracts, and he ate from their food. Jurists hold the view that it is permissible to deal with someone whose wealth is mixed, as long as it is not known that the specific item presented is unlawful.