You have the best intentions for you mother. I commend you on that. It sounds like you are young enough to still live in the house w/ her, so if they live together or get married, you will be in the mix...
You love your mom no matter what her decision, just like she loves you (even though it won't seem like it) when you make your own decisions.
It is hard to let someone else live there own life. But honestly, the ultimate decision of whether she is with him or not, is hers. :-( I know that sucks... but it is HER life... (and you are a very big part of it I am sure)
If he is divorcing his wife, maybe he does really love your mom?? I don't know. I don't have all the information and none of us really know what is going on in that guys head (or what he is even saying 1/2 the time from what you tell us... hehe)
Just be there for her... love her unconditionally... she does have to make her own decisions and her own mistakes... Just love her...