Especially if you're in a situation where you truly have no idea your partner would even consider cheating, being blindsided is like a cinder block of hot garbage in your stomach.
Of course, plenty of people have happy and healthy open relationships with clear guidelines and expectations. The worst part of being cheated on, though, is feeling disrespected, undervalued, or tricked.
There are a lot of differing opinions on whether or not it's your place to share with someone's partner that you know they're cheating, but it seems better than letting someone live a lie for years...
Without even reading it, I'm going to go ahead and say this person is NOT the asshole because looking out for a scorned spouse is never bad. Of course, it might feel like it's not your place to throw someone under the bus like that, but if you have knowledge that could save someone else from staying in a sham marriage for years on end and you keep it to yourself, then maybe you're even more of an asshole? It's complicated. Luckily, though, there's Reddit to guide us through life's turmoil...