Callie, wish I could promise you happy endings on this one - it's not an easy situation.
Blood is thicker than water but that doesn't mean we have to have people in our life if we don't feel they are good for us. My husband hasn't spoken to his mother for 12 year, Ar first it was about being hurt and angry at the things she'd done but after this amount of time he is totally reconciled to his feelings. They have both changed so much in the time they have been apart he feel the same way about her as he does any stranger.
As the others have already said your boyfriend needs to let his mother know himself what his decision is. It's not fair to expect you to be the messanger, especially at your work. A letter is a good idea - my husband did this, making it clear that he wanted no further contact with her and asking her not to use family members in this way as it caused them distress.
Best wishes.X