Norfolkdumpling, I'm sorry you are here w/us, but we have a great group of posters who are on the same path w/you and will be able to assist you along the way.
There is absolutely nothing you can do to reason w/him right now. He is thinking w/the wrong "head" and the infatuation has a very strong hold on him. Also, as his friend, she is validating him and his feelings about life in general. People who flip the switch and go into mlc, tend to look at us as the "enemy", therefore conversations w/the ow/om tends to valiadate the mlcer's feelings/thoughts. He sees her as a friend because he feels "safe" talking to her because she really doesn't know the true story of what your life was life before the BD. They are living in a fantasy world and can tell each other anything and they will believe it.
What you need to do is figure out what you want to do. Do you want to continue working at the same business? If so, you can do it, but it's going to be difficult being around him every day and trying to put on a happy face. Do you have constant contact w/him at work? How does his mother fit into the picture? Is she a supervisor or owner? If not, what she thinks shouldn't enter into your decision process as to whether you stay or go. I suggest that you find another position before walking out the door, if you can hold out that long. But, that's my two cents.
Go on your vacation and try to enjoy yourself. You'll have ample time to think about what you want to do. Just remember, when in doubt about something, do nothing. Sometimes sitting still and quietly will provide the answers you need.
The mlc journey is not for the faint of heart. It will twist you into knots if you allow it, but you also will come out stronger and wiser than before you began this journey. You will be surprised at how strong you are, but you first need to learn how to detach, focus on yourself, and the most important key...patience. You will not reach the finish line today, tomorrow or next week. It will take quite some to get there and we will be here to pick you up when you stumble and cheer you on.
First and foremost...take care of yourself, i.e., eat, get plenty of rest and exercise. You need to stay healthy to get through this.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.