Does a Muslim commit a forbidden act if he deliberately obeys someone who orders him to shave his beard, knowing that he considers shaving the beard to be merely reprehensible (makruh), while the one giving the order considers it to be strictly forbidden (haram)?

It is forbidden for a man to shave his beard, due to the saying of the Prophet, peace and blessings be upon him: "Ten things are part of the fitra: trimming the mustache, and letting the beard grow." And his saying: "Be different from the polytheists: let your beards grow, and trim your mustaches."

If your friend believes that shaving the beard is forbidden, then it is not permissible for him to shave it himself or to ask someone else to do so. Nor is it permissible for you to shave it for him, because this is considered assisting in a sin. Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." Rather, you must denounce him. So, for one who believes something is forbidden, it is not permissible for someone who deems it permissible to assist him in it; rather, it is obligatory for him to denounce him.