If you're sure she wants out of her relationship, and you do too....then I would just go for it.
If she dosen't return your feelings then you can always just say that you thought she might need cheering up. My friends (male and female) send me valentines cards out of friendship.
Word of caution though, don't embark on an affair with this girl...if she does like you then she will only find it offensive if you are in a relationship, and want her too.
If you want out of your relationship then then end it now...ther's no point putting off the inevitable.
Life is far too short to hide your feelings.I would send her something, but don't be pushy. If she likes you, she'll let you know. If not then just value your friendship.
Good luck.