Can you recover from chemotherapy complications?

My 75 year old dad was diagnosed at Christmas with non-Hodgkins lymphoma that had metastisized to the spine (but not the spinal fluid). He received six chemo treatments roughly three weeks apart. He got progressively weaker and by the third treatment needed a walker to get around (if at all), began to have issues with his bladder, needed several transfusions, and finally developed pneumonia and sepsis. The good news is they tell us the cancer seems to be gone (per PET scan). The bad news is that he has been in ICU, completed sedated, and on a ventilator for three weeks now. They have no idea when they will be able to bring him out of the sedation and off the ventilator. They are sure, whenever that might be, he will need at least a couple months in a rehab hospital. Has anyone else experienced these severe complications from the chemo? Any experience on how long it may take to start to improve? Will he lose cognitive ability from being sedated all these weeks?