My colleague has booked this afternoon off as she has a dental appointment?

[ChocLover]

Or so she says!
She left her email open on our shared computer by mistake with a clear email to her friend saying, yeah, the boss thinks I'm going to the dentist this afternoon, meet you in town at 1pm!
Do I :
A) Drop her in it and tell the boss
B) Tell her I know and blackmail her
C) Keep my mouth shut
D) Something else (Like what?)
She's a lazy moo anyway so I'd really like to get my own back on her for leaving me on my own this afternoon!!
drop the phone, I'll be doing that anyway!