You didn't have any grade-school crushes or "romances"? It's not a real boyfriend. They're not going to go to the movies or out to dinner. It just means that at school, they sit with each other for lunch and play on the playground. And because they're opposite genders and obviously have a crush on each other, they decided to call themselves a couple. Just remind her that at school, displays of affection are against the rules: No holding hands, kissing, hugging, etc. It's completely harmless! Unless they start making the physical contact above. We all had the grade-school "boyfriend" where I went, and it was just something for everyone to make fun of everyone else for. This is a time your daughter's developing her emotions and learning to analyze them in the context of what she already knows. My daughter's 7 and has had about 5 boyfriends since kindergarten. 3 of them are adults that she was crushing on for quite a few years. They were teen neighbors at the time, and in those few years now they've become adults. I don't worry about it because I know she's not going on dates and all. When she asks when she gets to actually date, I tell her it depends on her maturity and grades. So if I was you, I'd just ask what her boyfriend's name is, what he's like, why she likes him... It gets her thinking about what's important once you start probing the deeper parts of attraction and relationships. Oh, and ask if she knows how his grades are, too. She'll see that you're willing to communicate. That will be VERY important when she gets older and has a real boyfriend. Good luck!
M.