The best you can do I think, is to let them know your view. After that, you have no choice whether or not to let someone make their own mistakes - they will make their decision like it or not! You do have a choice to allow them to do this, in your mind. They may take your opinion into consideration, but in the end, the most peace you can find is not by them doing what you would hope for them to do, but by letting go of the worry and being there for them in case there are bad consequences, but the most likely scenario is that if there is a fight, it will be an unpleasant experience after which life will go on.
I think you will handle it well. I've read some of your posts and I think you are wise.