Is it obligatory to inform a friend about a forbidden act he is ignorantly committing, given the concern that he might not believe it or might take it lightly, which could then remove his excuse of ignorance?

Whoever sees his friend committing something forbidden is obliged to stop him from it and inform him of its prohibition. Otherwise, he would be sinful, due to the words of Allah Almighty: (And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful) and due to the words of the Prophet, peace and blessings be upon him: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." A Muslim should not refrain from speaking the truth for fear of his friend disbelieving him. Otherwise, he would be among those included in Allah Almighty’s saying: (Rather, they have denied that which they have not encompassed in knowledge and whose interpretation has not yet come to them. Thus did those before them deny. So observe how was the end of the wrongdoers.)