Yes, I think it's normal for this age. Unfortunately, my daughter dealt with it in 5th grade, from another friend who literally unfriended her for 6 months because my daughter played with another girl. Fun times!
So you need to work with your daughter on how to handle this situation and the other mom needs to work with her daughter as well. I have always told my children that they are to be friends with who they want when they want and no one can tell them who to be friends with. They don't get to decide who their friends hang out with outside of them.
Now that my oldest is 11 and in middle school, I'm seeing the girl who has been her best friend since they were 2 go in different directions. She is pressed to hang out with older girls and is trying to act so much older. Funny how my daughter knows that is wrong and we have been pretty lenient and trusting with her, but her friends parents have been very helicopter and limiting of what she sees and hears. Not saying our way is right and theirs is wrong, but allowing our daughter to run in to situations and learn how to handle them has proven to be very beneficial for her AND us.
As far as teacher involvement, I would ask her to keep an eye and let you know what's going on. Then you can work with your child at home. You can also ask her to let the other child's mom know so she can handle her side of it at home.