Father left mother without talaaq

As Salaamu Alaykum

My father left my mother 2 and a half years ago. He just packed up his clothes and left. He has not made contact with her since then, he however has not given her a talaq. What is the best way for her to end this marriage? He has not made contact with any of his kids or family. He has been involved and lived with non Muslims and might not be practising Islam any more.

Wasalaam