Now, I don't see Sven exactly blaming Lydia for things -- she's just convenient for venting. No, it's not mature, but it's a rare individual who hasn't done it at least once.
Again, this is BRAND NEW BEHAVIOR for Sven. Aside from getting slugged by his sister -- which might make him a bit surly -- he's acting rather like HIS feelings might be involved in this. Yes, he's dealing with the consequences of his actions, but these are brand new consequences. He's not trying to slide out. He's not hunting up a new woman to bed. He's MOPING.
It's like he's having an emotion about the situation. I mean, seriously. He could be just as angry at himself as Faye is angry at herself, but for him it is all new. I don't think Sven has ever been angry at himself for a decision. He's pretty much skated by dodging all that, not taking responsibility, not caring.
So, jeez, give him a little while to work all this out. This is complicated stuff he's learning.