Most people are under the misconception that if a Superman or Supergirl existed, that they would save everyone, all the time. They don't realize that even to them there are limits and they cannot be everywhere at once.

The irony that one of them would feel guilt and shame because she thought she should have saved everyone is a good marker that's she's still new at being Supergirl. Gibbs' having learned how to live with it, and showing her how he relieves his frustration establishes a mentor/student relationship she can't get from cousin Kal. This could be a springboard to some really good stories.