Any wisdom would be welcome.
You clearly care about your friend and have a lot of empathy for him. And because you care so much, you’re angry at the person who hurt him. But in order to offer true support, you’ll need to separate your anger from his feelings and allow him to have a different internal experience from yours.
As you well know, your friend is going through a very turbulent time, and what he needs most is to be able to hear himself—not you—clearly. This means that your main job as his friend is to be a compassionate and nonjudgmental sounding board as he sorts through his own feelings. To that end, what you need to communicate is not how you feel, but, using whatever language is natural to you, I care about you and I’m here for you. How can I help? Full stop. The full stop is hard—especially when you have such strong feelings—but doing so is crucial to his well-being right now, and his ability to arrive at his own place of clarity in the long run.
In cases of infidelity, many people have a hard time supporting their friends like this, because righteous indignation gets in the way. The thinking goes: This is a no-brainer. I have to save my friend from this selfish person. Or, My friend isn’t thinking straight. If this were me, I would leave the marriage.
But the problem with this way of thinking is twofold. First, although you may think you know what you would do in his situation, you don’t. Nobody knows what they’ll do in a particular situation until they’re actually in it, and you need to account for this gap between imagination and lived experience.
Second, only he knows what’s right for him. Remember, he’ll be the one to live with his choices, not you, which means he gets to choose whom to love and why. If he says “I love her” or “I forgive her,” and you try to talk him out of how he feels, he probably won’t want to share his true feelings with you, and your friendship will suffer. Your friend is going through a trauma, and what he needs right now is a friend he can trust with the full range of his emotions. Moreover, taking the position of “I love you but I won’t have anything to do with your wife” is akin to a parent saying “I love you but not who you love” to their adult child who just announced he’s gay and has a boyfriend. It’s hard to feel loved under those circumstances.
In other words, you say that you’re fiercely loyal to your friend, but being loyal doesn’t mean vilifying and shunning his spouse. It means allowing him to have his own feelings, which include great love for his wife, despite the deep pain this betrayal has caused. It means supporting his growth as he goes through his own process of rebuilding with her, and as the two of them try to understand what the infidelity meant, where it came from, and where they might go from here.