So there is a new best buddy in town (I waited a few days to make sure before posting). We'll call him "J". Fiona and J are always coming out of the school together and she is always talking about him. We HAD to invite him to the party (he barely made the list in time!) I volunteer in her class on Thursdays and she just couldn't stay away from him. He doesn't seem to mind. Her teacher raised an eyebrow at me at one point and said, "this relationship sure seems to be blossoming". Apparently what happened was that her teacher switched everyone's place in the circle and Fiona found herself next to J. She hadn't really interacted much with him before this.
He came up to me at lunch and asked if Fiona had told me his phone number. No. So he told it to me (and I am very impressed he knows his phone number; I wish Fiona did).
Fiona tells me they are going to be best friends forever.
Oh, and I couldn't sit next to her at lunch! She wanted me to sit on the other side of the table.