If one is sure that the choice will head towards disaster (which is often not easy to discern) then one should tell them explaining it logically. If they accept the suggestion then one should try and help them towards alternate ways. If they refuse, then it is best not to interfere and impose, unless one is responsible for the person as for example children or students. But one can pray and one should always pray. In our interaction we can try to see and show them the positives even when we leave them to their ways and the experience.
Detaching emotions is possible only when one can attach them somewhere, say with an absorbing interesting work or best, with the Divine. One should also disengage the different aspects of life that are dependent and grow more and more independent.
Affectionately,
Alok Da