My daughter is 7, there are two 8.5 year old girls down the street. Until this year they had nothing to do with my daughter, they felt she was too young. This year they use her as a pawn, when they are mad at each other they play with her, otherwise they don't. I've told my daughter that no matter how they treat her she should be a gracious friend, however if they hurt her feelings she doesn't have to play with them. She has figured out for herself what is going on and has accepted it amazingly well, she's happy to play with them when they want to and happy to play with others when they don't.
Encourage your daughter to be friendly. If she's happy with the lopsided friendship there's nothing wrong with that, learning to respond with kindness even when the other isn't kind is a wonderful skill to have. However if the friendship is hurting her feelings then tell her honestly that it seems like the friendship is one-sided and if she's not ok with that then your are ok with her not playing with the little girl anymore.
Remember that 7 year old girls are drama queens - they will love each other one day, hate each other the next, and be bff the day after that!