Are you sure she powered it up or perhaps she just forgot to shut it off???

I did that once and upon final approach into TPA, also about a minute or so before touchdown, the phone started beeping after catching a signal. It was embarrasing but purely accidental.

I would just let it go. If the same FA does it all the time, s/he will get in trouble at some point, but it is entirely possible it was just a mistake.