Yeah, I'd be annoyed.
If she's silly enough to get herself plastered all over facebook like this then I suspect it won't be long till her goose is cooked, so to speak.
That's what I was thinking too! But I'm pretty sure I'm the only one with fb at work. I feel bad for knowing, like I'm lying by with holding IYKWIM.
Also remember appearances can be deceptive. As someone who suffers anxiety that no one at work knows about. I have had to take days off for *me* time before (albeit few and far between) and she may have legit reasons that you would never know about.

Anyway just trying to give you a different POV.
That's exactly what I was after, thank you. I suffer anxiety myself so I do completely understand that.
As annoying as it is,( my boss does it all the time) she is entitled to the sick days and has a certificate. How she uses her sick days is ultimately her choice, and no one else can tell her what to do " while "sick."
Having said that though if you are supposed to be off with a bad back, and are actually off participating in physical activity much more strenuous than what work would be that can't be acceptable surely? The photos were of her running jumping, star jumps little agility type exercises...
Really she could've gone to her GP and got a certificate, if it were an anxiety issue and gone to the 12 week challenge, which would be a good thing to do to help with anxiety. A mental health week, I would completely understand that.

I think the funniest one I've heard about was a guy who faked a sickie, went to the cricket and then got busted because his boss saw him in a clip on the news... Karma can be something of a cruel wench at times.
Gotta love karma