I don't think it exists but it sounds like the best option for you would be a steel mug with a heated baseplate. The baseplate could detect the presence of a fu

I don't think it exists but it sounds like the best option for you would be a steel mug with a heated baseplate. The baseplate could detect the presence of a full mug with a force sensor and then start heating. When she makes her coffee she could place the mug on the baseplate and then keep it heated at her desired temperature. That way she could run around doing whatever and always have a warm coffee to sip on when she comes back to it.