If I had just read your question, I would have answered, “I cannot.”
Having read the details, though, my answer is different. You are not describing an unethical, immoral person. You are describing someone who is making a mistake. Everyone does that. Is it a bad mistake that will have long-term consequences for everyone involved? Yes.
Unless my friend had a pattern and history of treating other people thoughtlessly or acting totally selfishly, I might deplore their actions, but I would not end the friendship.
I would tell my friend honestly how I felt, which would be that while marriages can go wrong, seeking solace outside the marriage is not the answer. My best advice would be to break off the affair and work on the marriage. If the marriage ends, then to seek other companionship, but only after a suitable time.
If my friend chose not to take my advice, that would be their choice, but there would be nothing unspoken between us.