Three great responses already! I notice you're getting your masters in Education? Elementary? Secondary? Either way, you'll get to see this drama "live and in-person" quite frequently. Consider the advice you just got.
It's perfectly okay for you to tell your friend that a) you have heard her concerns, b) you're sorry but you haven't noticed the same things, c) you appreciate that she has mentioned it, and it will help you monitor the friendship so you can be there for your daughter if things do start to change.
It's quite possible that her daughter and this other girl simply don't mesh well... Nobody wants their child to feel excluded, but your daughter should be able to choose her friendships without pressure from someone else just because of that someone else's experiences. It's like when you enjoyed a movie and your best friend hasn't--it doesn't mean you're incompatible, you just took different attitudes with your experience.
Be kind to your friend, it's probably her way of letting you know that she's just as hurt as her daughter.