Does anyone know what would have happened to her if a flight attendant had seen her turn her cell phone on?Passengers are required by law to follow the instructions of crew members. Likely your friend would have been instructed to turn her phone off immediately. If she complied, she likely would just get a warning from the FA not to do that again.
If she did not comply its very likely she would be detained and arrested. They may wait til they land or you will find out pretty quickly that there is an Air Marshall onboard! Depending on how out of control it got she could be charged with numerous offenses (federal crimes!) and be banned from that airline. Some incidents of disturbance aboard a plane have resulted in planes diverting to remove the passenger - and courts have ordered damages to pay for the diversion. Not cheap I imagine!
Not worth it to check messages!