It Does Sound Like A Head Gasket Failure. The Gasket Was Probably Compromised When The Radiator Leaked Enough Coolant To Cause An Overheat Condition. I Think Your Mechanic Has It Figured Out.
His Description Is A Good One.
When the head gasket is breached, coolant leaks internally into comustion chambers and is turned to steam and goes out the exhaust pipe. Air is introduced into the normally “closed loop” cooling system and proper coolant circulation ceases.
What model-year and how many miles on this Nissan Sentra ?
CSA