Maybe Mayumi died too just like Fujiko's husband or forgot Fujiko because she had Alzheimer's or something given their age. I see part of her still longs for the free spirited woman she fell in love with years ago that's why she stared at her phone a little longer before picking it up realizing it's her son, just a little sad though they ended up for the reason of Mayumi sleeps with different people. She loves her husband but not in love with him because he's nice a man and treats her kindly with respect but to say she's happy is only half hearted.