Heat pump heating above thermostat setting

A friend called me and asked me why her thermostat read 75 but was set to 65. She flipped it to system off and it didn't shut down...same with flipping it to AC. She wound up flipping the breaker to shut it off (upstairs unit so she can live without it for a day for two.

My thought is to replace the thermostat and see if that takes care of it and if not, call a pro. I'm a remodeler and can replace a thermostat (if she can't) but would appreciate anything simple that I might be missing. Thanks!