I was just going to mentoin this. Even if she's suggested a date and claims to be moving, to my knowledge, in New Leaf you can still stop them.
In City Folk, Joey annouced he was moving in one week. I tried to stop him, but he insisted on moving. I kept pestering him, asking him to stay. At the end of the week (the day before he would be in boxes), I was about to give up. I went to his house and he told me something along the lines of; "Y'know, you've conviced me to stay. You must like me a lot!"
(The mechanics might've changed since though. I dunno.)