When I was a kid I had a dream where my boyfriend at the time and I shapeshifted into (non-anthro) wolves. I never made a wolf fursona, but later on when he joined the fandom, he did. That was a coincidence though, as the chances of someone choosing a wolf or fox fursona is quite high.
But yeah, she probably had that dream because she was thinking about the fox ears/tail she wanted to get. Dreams are basically composed of various memories and thoughts, often recent ones, rolled up into one nonsensical play. When I have a dream that sticks to me, I like to replay what I remember in my head and try to figure out what memories spawned certain details in the dream. It's pretty interesting.
And like Michen said, she really may consider herself a furry, though the dream isn't necessarily proof of it.
Oop, see a response as I tried posting this. Cool! I'm glad she's having fun with this, and I hope you two have a fun time at Anthrocon. (I'm kinda envious, lol)