What does the Church teach about indifferentism?

Growing up I was taught in Catholic school that thinking that any religion is just as good as another was the sin of indifferentism. Is there still a Catholic teaching on this? My good friend has become so fulfilled in his bible study each week at a Protestant church that he goes there on Sunday missing Mass and Communion every Sunday for months now. He sees Christ as the center and his Catholicism as incidental/unimportant. What is the proper/responsible way for me to respond to this? Is this OK to do in the Church’s eyes since he is still devoted to Christ? Or is this indifferentism? Should I just pray for him and accept him where he is right now?

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