Problem with daughter's friends parents

Ok Reddit, here is my problem. My 6 year old daughter (K) has become friends with a 9 year old girl (M) that lives close by. M's parents have a bad reputation, possible drug use and leaving her children home alone for very long periods of time. M keeps inviting K over to her house to play, and possibly stay the night. My wife and I are running out of ways of denying them without blatantly saying the real reason we object. Any thoughts on how we can handle this? M seems like a nice girl, and they get along well. We are fine having her over at our house, even staying the night, we just want our daughter to stay away from M's house. Thank you for any advice you have. via /r/Parenting http://ift.tt/2amD8cR