Please note, while I am an ordained minister.. it's one of those online ordainments, I just took it further and got licensed by the state. I can legally solemnize marriages, but I have no religious (or moral) training. Hence the question.
You're way too involved already. At the very least, I would not marry them. I would want a friend to step in but clearly he is not of the mindset to listen. Unfortunately, there's little you can do. If you continue to push he will choose her, they always do.