I have an INCREDIBLY hard time believing that Sam would even think about going through with something that Eva hadn't green lit. These people are married and he's never given any indication that he's the type to do something that heartless.
I have two theories as to what might have happened: while Sam was busy with this girl, Eva went out and met her friend, fucked him, came back and had 2nd thoughts about the entire night. OR, she possibly may have read the comments on this board and wasn't too happy to hear about how Sam fucks this girl better than he fucks her.
Truth be told, I'm hoping that none of the theories are correct and that she was upset because of something totally unrelated.