How to deal with a monster

When I was between the ages of about 7 and 10 I was sexually abused by my mums boyfriend, not just fondeling, I went to the police once when I was little but nothing was done because of lack of evidence and kids sometimes are told to make things up during custody battles.
I only started dealing with this issue about 4 years ago, 15 years after the event, so I see it all for what it is, as before my councilling I was in denail of the whole situation and could just bare to be in the same room as this monster, now that I have dealt with this it makes me physically ill to even think about being in the same room as him.
Now 20 years later I have this situation that I will have to be in the same room as him as my mum is still seeing him, behind he husbands back, he is her date for a family function, after all these years I thought she should know, she I told her about it and she didn't believe me, as she has always put herself first, and after telling her all this she is still choosing to parade him in my face.
I am in a difficult situation as I work with her and it's a great job, will not find another like it, so it's a little hard to conflict with her, and as I had already confronted her I don't see what the use is as she didn't believe me the first time.
How do I deal with the situation when it arrises, not going to the function is not an option...