Is it considered a forbidden act in Islam, incurring sin upon the doer, to feed a Hindu friend beef without his knowledge, while being aware of its prohibition for him?

A Muslim should choose righteous and pious individuals for friendship and avoid associating with corrupt and misguided people. There is no harm in eating with a disbeliever if the food is permissible. Regarding the beef mentioned, which he ate unknowingly, there is no sin upon you for that. Rather, the sin falls upon him for prohibiting what Allah has made lawful. Sin would be incurred if he asked you about the meat and you told him something other than the truth. The sin is not in feeding him beef, but in lying to him.