My 84 yr old father had a mediastinal lymphoma, had 10 days radiation, 25 days out still with nausea, fatigue. Is this to be expected?

Yes, common. These are common symptoms of radiation treatment and chemotherapy. Be very supportive, have him keep follow up appointments, receive good nutrition, plenty of water. These treatments slow our ability to heal. They can be life saving though, take the good with the bad. Hope for the best.