Is it permissible to borrow money from a friend whose wealth is likely a mix of lawful and unlawful, based on his work as an agent for a foreign company and his receipt of a percentage?

There is no harm in dealing with a friend, accepting his help, or borrowing from him. There is no need to investigate or scrutinize the source of his money unless it is known that he acquired it unlawfully. Investigating such matters is considered an blameworthy affectation and an innovation in religion. The basic principle regarding a Muslim's wealth is permissibility, and what is unknown is as if it does not exist. However, if a man is known to have unlawful elements in his wealth, then dealing with him should be avoided out of piety. As for the discreet Muslim, there is no doubt in dealing with him.