Mel -
It isn't unheard of for juniors to get bids. Being a transfer may be a bit harder if she doesn't know a lot of people on campus. BUT, if she is open to all of the groups, she would fare better than a junior transfer at some other schools. She could beef up her chances by getting recs and making sure the chapters know about her good grades and such. I would think her chances would be much less than a freshman or sophomore, but still not an impossibility. She'll just need to work harder (recs and such) and accept more cuts.
See
KiwiJuice's recruitment thread from last year. She was a junior transfer. And
this thread as well.