How can I stop this relationship from happening?

maybe you could write her a anynomous letter stating why you feels this person isnt good for her..maybe it will give her something to think about....or just trying being honest with her,,if she's your friend she will always be your friend but ultimately the decision is hers..if she follows through with it ..at least she was forewarned...some people have to experience things the hard way..hope it works out in your favor!