His Mother is Ruining Our Relationship

I am in a relationship where my fiance's mother is a narcissistic/obsessive-compulsive person. Her actions have reeked havoc with his 3 year old daughter. It has gotten to the point where I refuse to go to the house to visit. If she isn't throwing a plate at me she's telling me that my fiance sees other people and if I knew what was good for me I would leave him. Which none is true. No one is better than her to take care of his daughter, she complains that she talks about me too much, that she talks about my parents too much, she complains that my fiance's child support is paid out of a joint account and has riled up the mother of his child so much that now the relationship is a problem to her. His mother badmouths him to his clients of his company and to his friends parents saying how ungrateful he is to her and that it is acutally her that does the work for his company. He's in construction. She prides herself of the fact that she successfully has run-off every other girlfriend that he has had and tells anyone who will listen that she knows that this relationship is doomed and we aren't right for each other and that she knows it for fact. Not to mention the language that she uses around his daughter. Telling her to tell Daddy he's an 'F'n Dope' and things of that nature. I do not know how to deal with this anymore. I find it difficult to suggest to my fiance that he should visit his mother. Everytime he comes home, he's crying! I do not go, I fear for my own emotional stability not to mention his. But I refuse to say anything to the effect of 'do not go' because if anyone is to drive him away I would rather it be her. I just have no idea how to deal in the meantime. Please advise. I do not feel I can marry him until we find a way to deal with this issue. Thanks for listening.

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