i don't know how old these girls are, but in general, kids, and especially girls, can be worst enemies one day, then bff's the next. they say things like "i hate you!" and not only do they not get what they're saying, they forget they said it an hour later. i understand it bugging you. i would keep an eagle eye on her and be very "present" so that she knows she's being supervised.
i had this problem with a neighbor girl who liked to boss and bully my (younger) son around. i said something to her about how that's not how we play with friends, and if it continued my son would have to come inside. i made sure she knew that i meant business - then stayed visible when they were playing together. she would look furtively towards me as if checking to make sure i was still there...then eventually she lost the attitude (i believe her home life was not great and it was less to do with me, and more to do with things settling down for her, but regardless, i wasn't going to let her take it out on my son). she was 7 and he was 5 at the time.
you are an adult, and this is a child. be friendly, fun, and keep it light, but if she acts out, don't be afraid to be firm and in charge. you need to be anyway. you're not going as a playmate for them. you're going to help supervise.