What can I do when she's like this?

My girlfriend's parents were very abusive in the past and she is doing well now and goes to therapy. This past week her parent's sent her a valentine's card and any time she sees them or hears them or gets something from them she loses it. She just gets moody and really sad. She has been moping around a lot this past week and wearing a hat to cover up how sad she is when she goes out. What can I do?