A big no-no is pointing out the attractiveness of others. For example: "We got a new receptionist today. She's really pretty, so I wonder if that's why the boss hired her."
Also, never agree outright with a negative self assessment. For example: "Ugh. My hair is just awful. Don't you think?" Tread very, very carefully here.
As to your predicament, that was a pretty low blow on her part. However, it's not totally inexcuseable, in the sense that forgiveness is how strong relationships last. A single insult, borne of frustration should not be the end of things. If this were a pattern on the other hand...
Completely aside, and without trying unduly influence you here,
"I'm thinking about letting her do better." is an excellent line for a movie
