Is it a problem that she is with someone or is it a problem that she is with him.
Basically what I am asking is- Is it the idea of your mother being with someone that you don't like or is it the person your mother is with himself.
If it is the idea of her being with someone then you need to get over it. She can't be single forever just because you want her to be and if she is not with this man, she is just going to be with someone else. One day you are going to leave home and not see your mum as often and when this happens she will be alone because you wouldn't let her find happiness.
But if it is the man that you don't like then you should explain to her the reasons that you feel he is not a good fit for your family and then leave it alone. If you keep talking about it, with your mum or anyone else, you will just end up making problems when there aren't any and not liking someone can become an obsession that you can't get past.
Once you have clearly stated your feelings to your mother, verbally or in a letter, it has to be up to her to make the decision for what she feels is best for her life and happiness, and unfortunately this may not end the way you want it to. This situation would be different if there was actually something wrong with the man (for example he was abusive in some way) but you have already said that he treats her right so I don't believe there is a big problem that you need to address with her.
Remember what people say about teenagers - the more you tell them not to do something the more they are going to do it? Yeah well, this applies to parents too so don't keep talking to her about the same thing over and over again.