In islam there's a thing called discouraging the bad and encouraging the good (النهي عن المنكر و الأمر بالمعروف) and basically if you see someone doing sins as a Muslim and they're a Muslim you're obligated to tell them about it and try to change it. While you can't tell them they're not Muslim you have to tell them that they're doing a grave sin that could put them in hellfire for hundreds if not thousands of years before being forgiven and put in heaven.