If she has your pictures, you either didn't erase it properly, or the sd card was not connected when you erased everything. Or, if she is really evil, she downloaded a tool that gets back deleted data from the datablock. As you probably know, deleting files actually means that the disk will, instead of actually erasing blocks from disk, give the block free to be written over, which is why tools can be used to get (some of the) data back. Assuming that she would have done this though is quite a stretch, but I'm just mentioning it as a possibilty.
Let that also be a lesson: Always check at least the /Pictures, /DCIM and any application folders you may have with personal data (/Dropbox I guess) and delete the data you may find manually. If you want the files to be unrecoverable, apps exist that actually overwrite "empty" space, like Secure Delete Android (haven't tested it though). See http://www.securedeletion.com/
At this point though, the only thing you can do is ask her to delete the pictures and hope she is a thrustworthy person.