hello starfish,
the more you can stay uninvolved, the better. i went through months of checking out stories, putting clues together and "busting" my abf, because i couldn't bear not knowing the truth. the fact was, it did no good for him and no good for me - my nerves were shot.
it is true that your mother is in some kind of denial. she will either come to the painful truth (meaning truly understanding what is going on) or she will not. it is, for now, her coping mechanism.
the more you surround yourself with supportive, educated folks, the better for both you and your mom.