I travel with a heating pad and have had no trouble with United and Air Canada flights (MN & CA) in the past 5 weeks.
Hope that helps.
Worst comes to worse, TSA open her suitcase, see it is an electric blanket and close up her suitcase. No biggie. Stick it on top so they don't have to rummage through her stuff (or put things grouped in large Ziplock bags so nothing falls out by accident.)