She is not required to file a joint return with him and does not have to cooperate with providing him any social security numbers. If he is violating the protective order by contacting her she can report him to the police.
She and her child may possibly be her mother's dependents. As far as the child is concerned, unless the two parents combined provided more than 50% of the child's support, the child is not the dependent of either parent. Therefore it would boil down to dad's 4 months of providing full support, plus his child support for the rest of the year, vs the support that grandma provided for 8 months of the year.

