hi your daughters friend is the person who deep down has the problem within her self not your daughter, inside that little girl kersten is proberbly having more emotional problems that you cant see on the outside,she also knows that your daughter will always be there for her no matter what therfor she is able to manipulate her, i would take this in my own hands considering the age and go and speak to the childs mother first on the quite and explain the situation and how its effecting your daughter, i think kirsten needs to be explained to about how to involve other girls and to be told that its ok to have more friends together at certain times and that it can also mean more interesting play within a group and not just one to one, is there also any other girls or relations you could involve with your little girl to help her confidence,i would certainly go over and let the other family no what there girl was doing if it wa mine daughter i would class that as a type of bullying and get it nipped in the bud straight away.hope it all works out, you really feel for your kids and want to protect them when something is bothering them it bothers you, and its not like your daughter can really say well stuff you il go and play with someone else and thats the hold that kirsten is using on her soit needs a different aproach and talking about it and making the other girl see will be the only option.