No, unless if she asks for your opinion. I think it would be best not to mind her business so that she will be offended. Of course, as a friend you are thinking what is good and best for her but sticking your nose on her private life will only make her defensive and she might think that you are being judgmental. Usually people who are in love or who think they are deeply in love cannot think clearly and rationally and have the tendency to be close-minded. Anything you will say against the relationship or the man she will take it as a threat to them, so better hold your tongue until the right time comes.

But if she will open up to you and ask for your advice, that is the time that you will give her a piece of your mind. Maybe she just needed some time to think and confused and this will be the appropriate time that you will tell her that having an affair with a married man is no-win situation. At this point, she will be open-minded to your advice or suggestions.

But in spite of your advice she will continue the affair, that is already up to her. At least you have done your part and advice her against continuing the relationship. The rest is up to her if she is going to listen or not, anyway whatever her decisions, she is the one who is going to face the consequences later. You can just pray that she will be enlightened and decided to do the right thing, that is to cut the relationship.