My best friend has told me that he is sleeping with a 40 year-old woman who he met on the internet and who lives 180 km away in Silesia... I have no idea what to make of this.. She is his intoxication, he wastes all his money on her. I do not want to think about this but perhaps it is better not to judge, or is it?
If you are really close, you can, in an warm way, and stressing it's your opinion, comment with him that this is not a healthy relationship and explain why you think so. Don't press the issue, don't insist if he says he doesn't want your opinion, and, in case he allows you to explain why, pay attention to his reactions as you speak to know if it's not turning against your desired aim which is to be heard.
Don't return to the subject if he doesn't listen and, as a friend, be prepared to be by his side when things start going wrong. In the meantime, don't show your opposition to it or to his "friend", but don't force yourself to show support.