I've been in Lydia's situation before and it is incredibly shitty. If you say something, you risk ruining somebody else's relationship. But if you DON'T say something, it poisons your relationship with those same people.
I'd rather err on the side of honesty, and have in the past.
There's one worse: being given such information by Party S, biting the bullet and keeping your lip zipped, and then finding out that Party S has told everyone he knows except Party F, the person that it matters to, and that, by playing the odds, has managed to get somebody to finally spill it to Party F, which was apparently the plan all along. You're left with the Charlie Brown question: are you a hero, or a goat? Me, I felt like eating laundry, stinking, and shitting all over the place*, i.e. decidedly goatlike.
And by the way, for everyone wanting to slam Lydia, there's a decided difference in taking someone into your confidence for help in working something out and casually dropping a stink-bomb the way Sven did.
*Well, verbally shitting.
Edit: Which, I realize I didn't say, is what I think Sven would do if he could. Drat Wil for being on a road trip!
Why would it be "biting the bullet" not to say something you'd have to go out of your way to say? Do you really think Sven is going to tell everyone except Faye?
I guess it's my advanced age -- I've been through this particular situation from a few different viewpoints over the years. There are no good scenarios. I've been the one told I was being cheated on (I didn't want to believe it. Once it turned out to be a lie, and once it turned out to be true. Both times, the drama was large, the relationships were ruined all the way around, and it was ugly). I've been the one who told about a cheater (same scenario, only I got something thrown at me) and I've been the one who told the cheater I wasn't carrying his news for him, he better do it himself. (The only one where all things DID NOT go up in flames, although there was ugliness afterwards). Lydia isn't even Faye's friend with some sort of loyalty involved. She's met Faye, what, two or three times in great awkwardness?
Here's a thought -- perhaps there will be more conversation and Sven will talk this out and come to a decision. Personally, I hope he will be an adult about it and do the telling because it's the right thing to do, and that Faye will handle it non-violently (sorry, I don't think violence is an appropriate choice in this situation -- funny, yes, but it is just another way to control people -- do what I want or I will cause you pain).
However, I will say that Jeph has done his job as writer -- we are obsessing about it! Hell, I'm on vacation for my birthday and I'm obsessing about it!