From my experience of raising three girls, I know that girls can be, as said previously, friends one day, and enemies the next, and friends the day after that.
It is easy to get roped into squabbles, but it's also inadvisable.
I think the advice offered is sound - have a chat with the other mum, who is concerned for her child, and nothing wrong with that, and keep everything light and easy and try to encourage her to see that spats are part of growing up, and adults getting involved solves nothing at all.
Good luck.