2000 chevy blazer overheating

this last weekend I replaced the thermostat and heater core hoses because
one hose had a pin hole in it. while the old hoses where still attached to the
heater core i used compressed air to reverse flush the heater core and to
check for leaks. then flushed several times back and forth between the
hoses with water until water was clear. then installed flush kit, removed
thermostat, put housing back on and flushed the whole system until water
was clear. then installed new thermostat and refilled the system with new
coolant (green kind, not dex, i heard dex can case issues in older vehicle)
checked for leaks again. and started vehicle. let run with heat on until
operating temp. then ran at 2000rpm for 5-10 minutes. no problems. drove
the vehicle back to its owner 10 miles away, no problems, then today owner
sends me a pic of the temp gauge in the red. any idea of why this is
happening?