I know of a man who is 23 now. He has been in love with my 16-year-old friend since she was 13. He is married, but is still in love with her. She loves him too, but knows that it is wrong. They both know. But they can't seem to want to part. They haven't had sex or anything, just shared a few kisses.
Is it wrong for them to be involved? By law, yes, but if they are in love? He would give anything to be with her but getting a divorce in his situation is a lot harder than a normal divorce. He wants to be with her even if he is married.
I will take your question at face value, that in fact you are friends with these two people, and not one of them pretending to be someone else.
The real answer is that they will do what they do. They will reap any benefits, and suffer any consequences. You cannot run their lives for them. Even if they ask you for advice, it's best that you ensure they make their own decisions. Often, when you tell someone they should or should not do something, the advice will actually push them toward the other choice. The best way to answer a request for advice is to ask clarifying questions, ensuring that the people involved get in tune with their values, how they would like to see their lives in the future. So, if for argument's sake the 16-year-old girl asks you what to do, ask questions like this in return:
"Suppose he gets a divorce and the two of you eventually marry. What will life be like in 10 years' time?"
"Suppose you decide to just stay friends. Do you think you will both fall in love with someone else after a few years?"
"If he partners up with you, will his current wife get over it eventually?"
"If this someone else was facing this problem, say your twin sister, what would you advise her to do?"
"If it was your twin sister and she chose to be with him, would you approve of her?"
"He must have been in love with his wife when he asked her to marry him. Now he is over it. Do you think the same might happen with you?"
And to balance this: "Suppose you decide not to break up their marriage but go your own way. What is the chance that you will never find love again?"
Try to ask questions that are balanced, so you are not pushing her toward one decision or the other, but explore the personal consequences of her actions for herself and others involved. One helpful trick is to design a film script for yourself. Set out a character so well that an actor can do it exactly as you intended. You can suggest to your friend that she designs the person she would like to be, and then step into the role. No one else has a say in what she designs. If her values include doing no harm to another person, she will choose to avoid breaking up the marriage. If instead it is to follow her heart regardless of consequences, then she will allow the relationship to progress.
I hope this is of help,