Looks to me like Sven only told Lydia because he was distracted by his self-angry thoughts, I think it just slipped out. We can see here that Sven's relationship with Faye, whatever form that might take, means a lot to him because he is normally so open about his 'encounters', but he wants to keep this one quiet so he can continue meeting with her.
We also learned that Faye is better at the sexin' than Gina Riversmith
I"m leaning toward this one. Sven is having brand new thoughts and they are taking up all this thinking power. He can't think about this new sensation of guilt, the fact he actually didn't like the sex he had, and that he might have feelings for Faye (who, from past evidence, would never admit having feelings for him) and keep up a subterfuge.
As for the intern -- it really is none of her business, even if she knows. Not everything a person knows has to come spilling out of his/her mouth. If asked, she need not lie, but to threaten to run and tell isn't about honesty, but about power. She's making a threat to someone else who has some power over her (her boss, so to speak) and now she has something that gives her power over him, so in typical immature style, she's gonna tell! she's gonna tell! She's just short of dancing around and pointing her finger.
How do I know? I've been in a similar spot at the intern, knowing a friend was cheating largely with his then-fiance, also a friend. I was never asked, but I never threatened to tell. I encouraged cheater boy to tell, though, and expressed my disapproval, but in the end it isn't my job to go around forcing my moral code on others. They gotta make their own mistakes and live with them.
Wow. I just realized Jeph has finally created a character to whom I have taken a strong dislike. Whatsername intern girl was not my favorite when she appeared, with her unrealistic expectations and somewhat obnoxious expressions, but now she's just taken the next step.
Why would he want to boast about someone who's lousy at bangin' knowing full well he'd be killed for it?
I didn't get the idea Sven was boasting, really, and you point out two excellent reasons why that interpretation doesn't make sense. I really see it as Sven having Brand New Thoughts, and just having them is really shaking him up.