Actually, I agree with you: but I suspect we were supposed to imagine Usha as being so shocked by the news that she's not just giving advice: I think she's always been rather envious of Ruth for having a stable and (apparently) strong marriage and finds it very unsettling to see it possibly break up. She was, for instance, quite wrong that Ruth was 'perfectly happy' before Sophie came on the scene - but it's interesting that Ruth didn't try and correct her on that, for all she wanted to defend her conduct.