Capital Gain

My father is 74 years old and lives in California. He is selling his property that he was renting for the last 20 years. He will be selling it for $320,000.00. He will be receiving about 289,000.00 cash. My questions is, will he need to pay taxes on on the amount he is receiving? If so, what would be the estimate he would have to pay? He is retired and hasn't filed taxes in the last 10 years, or longer. Thank you