There is nothing illegal about an employee of a company selling goods or services to his employer, either directly or through a company he owns, or steering business to a friend. Your friend cannot do anything intrinsically illegal in doing this--e.g. he can't conceal his relationship wth the supplier from his employer or any terms of the transactions, since such misrepresentations could be fraud--but as long as he is open, transparent, and above board, it is legal. It could still be, of course, against company policy--many things that are legal, such as dating a coworker, are against company policy--and if his company has a policy against this or he has a supervisor who objects, they could refuse to let him do this or even fire him, but that's because companies have broad discretion to set policies, to terminate employees, etc. From the law's point of view, though, as stated, this is legal so long as it is done in a transparent, legal way.