2001 Subaru Legacy "check engine" light and high

the check engine light goes on for a number of things. coould have picked up a diferent reading from changing the octane. If you hear anything unusual or see anything unusual then i wouldn't drive it until the appointment.

i would say since it happened right after she got gas that maybe the fuel cap isn't tight. typically this is the only time i would suggest this.

high octane fuel only elevates the pressure the fuel will pre-ignight at for high compression ration engines. In a standard compression ratio engine the fuel has no real difference. (It's only gas company marketing that makes people think it has some other benifit.) The light is likely something else and there is no cause for alarm due to the gas. You should go to the shop and have the code checked, a loose gas cap can do it.