She's probably upset because it is embarrassing.
It might just be that she's embarrassed that a boy likes her. However, you also have to consider what the other girl's motivation is for delivering this "innocent" message. I can see a few possibilities:
1. The boy asked her to ask your daughter if she'd be his girlfriend, and she's just helping him out.
2. He asked her to ask, and she said yes because she wants to get involved and stir up drama.
3. He's popular, and the other girl is trying to set your daughter up for humiliation by making her think he likes her.
4. He's unpopular, and the other girl is trying to set your daughter up for humiliation by getting her to say she'll be his girlfriend (this happened to me in the fifth grade).
In the short term, I would reassure your daughter that she doesn't have to be this boy's girlfriend just because he likes her, and tell her that she can use you as her excuse if she needs to. ("Sorry, my mom won't let me have a boyfriend until I'm in high school.") I would also try to find out about the other girl: are she and your daughter friends? If not, who are her friends? Is she popular? How is she connected to the boy? Have she and your daughter had a conflict in the past? At this age, I think 90 percent of boy/girl stuff originates with the girls -- most boys aren't really that interested in girls yet (although they might enjoy being fought over), but the girls will use the boys to try to get status, stir up drama, or hurt each other.
Good luck -- let us know how it goes!
Thankfully, I haven't heard a thing about who's popular and who's not, yet!!
I'm sorry that your daughter is upset about this.
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