Mrs. Reed has always been somewhat delusional where her son John is concerned. Her coddling of him when he was young and the complete blindness with which she accepted his utterly selfish nature evidenced an unnatural capacity for self-deception. The stress of John's demands on the family finances over the years and the shock of his suicide cause his mother to suffer a stroke, and when Jane meets with her, Mrs. Reed is not completely lucid; her "wandering look and changed utterance told what a wreck had taken place in her once vigorous frame".
When Mrs. Reed talks to Jane now, not only does she speak of John as if the "poor boy" is still alive, but she also speaks to Jane in the third person, talking about Jane as "that child". Her long-cultivated blindness towards the truth where John is concerned is now exacerbated by an inability to distinguish present reality from memory because of the delirium caused by her illness. Another contributing factor might be that it is just too difficult for Mrs. Reed to accept the horror of John's death and the monstrosity of her behavior towards Jane. It is easier for her to deny to herself that her son is gone, and to never acknowledge Jane's current presence when speaking about the past, distancing herself even further from reality (Chapter 21).
Mrs. Reed's son was her favorite child and heir to her property. She has still not come to terms with his death and, therefore, talks as if he is still alive. This is similar to her behavior when he was a child. She ignored all of his bad behavior and blamed Jane for many of his deeds. He was able to totally manipulate his mother so she never gave him the discipline he needed. When he dies, she is so filled with both guilt and grief that she cannot accept his death. She is also dying herself and has kept the secret of Jane's inheritance out of spite so that adds to her sense of guilt and shame. Had she accepted Jane and disciplined her son, she might not have had such a lonely death.