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I'm probably completely wrong but I thought it was possible that he didn't know she did it. I felt that after what she said, Walter wanted to protect her (the piano teacher) because she was being so selfless and fragile at the same time. At that point, I don't think he had any clue how messed up she really was. When he went up to the bathroom to seduce her, I believe he expected a "normal" type of sexual conquest.
I can see it from the other perspective as well though.
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Yep.. I think he knew it was her.

This is not really answering your OP but I thought this scene was exquisite - one of those rare moments in films where the character is so finely captured. The way she delicately wrapped the scarf around the bottle and then gracefully deposited the broken glass in the pocket of the coat before placing the scarf back on the coat rack but in the wrong place. Just loved it. I'm a Dario Argento fan and I could imagine this scene in one of his films.
I'm a fountain of blood
In the shape of a girl

Of course, he noticed her leaving the room right before while they were on the piano, or coming back, I'm not sure which. He was flattered at the time when he realised, that she would do something that extreme out of jealousy in a way, because he didn't know what would come after.

btw..thanks Lemoen for asking the question, I was also curious as if he knew Erika was responsible.

The look on his face indicated he did know it was Erika. What's interesting, he wasn't turned off by her actions but actually wanted her afterwards. Did he like that she was jealous or sadistic? For those that read the book, did he know Erika was responbile for the incident?

I think Erika's sudden strange reaction and what she said made him realize that it was her.
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