What do we say when childs mother.....?

Our (age 11) son's (my stepson's) mother has a drug addiction and can only have supervised visits with her son. She does not take responsibility for the problems her addiction has brought about. She has talked her supervisor into letting her take the child off alone and so we are not allowing visits through that person but have offered visits at the park with our supervision,which she only did once . Now twice she called and asked if she can have a visit (without her supervisor) and when we say no she will tell son (on the phone) that we said no, making us look like bad guys. She only calls about every six weeks, has a different boyfriend every time we see her, has no job, and tells son that it isn't her fault so that she gets to be the poor victim who is doing the best she can. Child still doesn't realize that his mom is being manipulitive and hurtful to him. I can only imagine how he feels about all of it. What can we do to keep from being the bad guy and still keep our son safe?