There is an essay around here somewhere (someone will be able to point you to it, I hope) about how socialization issues with gifted kids are often a matter of their being ahead socially, rather than behind as is commonly thought.
The problem is that they have higher expectations for friendship than what the other kids their age have. They expect to be able to hold conversations, be treated in a certain fashion, and the other kids aren't there yet, so they get frustrated (and/or the other kids do) and it comes across as being socially inept.
As for what you can suggest, can you not suggest puzzles, Legos and computers? Or is that not an option with Montessori? I've heard so many mixed things about it, but I'm not familiar with it myself. He's probably not engaged with what she wants him engaged with because it bores him to death.
