Praise be to Allah.
This person has no ties to this family, whether through blood or through breastfeeding (radaa’ah). On this basis, if he is a mature and responsible adult, it is not permissible for him to mix with them and look at that which is forbidden. This is the case if he is still Muslim, let alone if he has left Islam.
So it is not permissible for her to shake hands with him, or to be alone with him, or to uncover in front of him, because he is not a mahram. (See also question # 5538). She should not greet him with salaam nor return his greeting so long as he is a murtadd (apostate). We ask Allaah to keep us all safe and sound.