I definitely agree with the Yote. I would:

1. Prepare amendments for each as single. The girl friend should be contacted if she is not aware of the situation and should be allowed to participate if she chooses assuming that she signed the return and shared the refund.

2. I would submit the 1040Xs with a copy of the notice and a check for the (full) difference. If the girl friend is not participating I would consider submitting an unsigned but watermarked 'draft' with a note giving whatever explanation for her failure to participate is appropriate.

3. I would give Dad a large invoice and a larger stick to knock some sense into his kid.

edit: You might also consider including a 2848

.