Need advice please! 8 year old living with a meth addict

Has your husband/ SO ever done something or made a parenting call that completely changed the way you view them?

My fiancé and I have been together for almost 4 years.  We have a little girl together and he has a son from a previous relationship.  We obviously have our daughter full time, where as we only have his son on the weekends.  His son’s mother lives about an hour and a half away from us so she gets support from my fiance’s grandmother and they share time with the son.  Well, my fiance’s MOTHER is a homeless, unstable, meth addict who bounces around from house to house.  We let her stay with us a while back while she was saying she was getting clean, but I found out she was still using and stealing from us so I kicked her out. 

She now will stay with her druggie friends or grandma.

My issue is-  no one seems to care that this drug addict is constantly around an 8 year old boy.  When she is living with the grandmother, my fiance’s son is also there.  She takes him to do things, drives him places, and there are just no boundaries.  When I find out she is with his son unsupervised, I ask my fiancé if she is clean.  His response is “I don’t know, or she used a couple days ago. Or you don’t understand meth- it’s not dangerous unless she doesn’t sleep.”  LIKE WHAT!  This is not just some pot smoking on the weekends, this is METH and his son being constantly exposed to someone who is on it and no one doing a damn thing to stop it. 

It has gotten to the point where I seriously am questioning his judgement as a father and being with him.  My worry is, that if I leave him- he will expose our daughter to this woman as well!  I don’t know what to do.  Any advice would be so helpful…. I am at a loss.