The answer to this question can be found in Chapter Seven of this great novel. John describes the way that he was able to leave his house easily in order to meet Lorraine and go to Mr. Pignati's house. The poor relationship he has with his father gives him ample reason to be able to leave home and walk off in anger, and this is what happens when his father, nicknamed "the Bore" by his son, starts getting at John over the dinner table. His mother's normal reaction to such events is to leave, as is shown when John explains her actions:
"I have to get the dessert," the Old Lady said, violently polishing a teaspoon and dashing out to the kitchen. She always gets terrified if it looks like my father and I are going to have any type of discussion.
Thus, ostensibly, John's mother leaves the table to fetch dessert, but really, we know that she cannot handle any overt conflict between her husband and son, and as a result leaves the table to avoid witnessing yet another fight between them.