There are many times when WE are so sure THEY are making a mistake and yet in the end, their choices turn out just fine. It just happens to be a totally different choice than what we would have chosen for ourselves.
If THEY are discussing the issue with you, then you can offer a suggestion in the form of “may I offer a suggestion?” and then offer it in a such a way “you might consider …….” If you push an idea or tell them they are wrong they will most likely tune you out.
In the event that they make a bad decision and it turns out badly be careful not to sing the I told you song or the “you should have asked me song”. The last thing they want to hear is that you are smarter than they are. The best thing you can do is be supportive and say ” everyone makes mistakes, that’s how we learn. Where do you go from here?