Erm... I feel like I've stumbled into a twilight zone...
Why would she have written anything to wind up someone she's never met?
Why is it so completely out of order for a woman to write something saucy in the card of a married man, so much so that there's all these faces around and even someone advocating assault!
If something had happened then I doubt she'd have written anything in his leaving card ffs, AND I doubt he'd take it home to show you either.
Before you head round to slap her consider this scene: there she is sitting there, racking her head for a witty/funny thing to put in the card as it makes its way round the office? And decides to say something about when Jerry frm the post room made a rude crack about the graze she got on her knee. Jerry leaves and she and your dh have a joke that means nothing to your dh, but made her feel less upset by Jerry's lewd, sexist comment.
Or maybe she did trip over and make the joke herself
The point is there's many more possible reasons behind her comment than the long shot that she's either having an affair with your dh, or (not so) secretly wanting one.
If you are seriously upset about this you need to have a real think about your relationship as a whole and what about it is not up to scratch, as fury over this can only be understood as being a straw breaking the proverbial camels back.
Not that I think you are a camel, in any way