Answer:
It is his mother and she clearly meant so much to Morrie. There are many things to take into consideration - why is someone still mourning when it has been over 70 years prior. Does he feel guilty about something? Perhaps he didn't get to do something with his mother that he might have wanted to. Perhaps he wasn't there to say goodbye to her when he would have liked. People sometimes feel guilt when they are unable to let go of something. As it's been over 70 years, you would hope this person would have accepted their loved one's death.