WOW, what a situation.
I have never been in this type of situation, but as a friend he should be willing to help you out.
Tell him you are trying to advance your own career, with this latest book ask to be on the cover, ask him to let the publisher know that you were partners in the last one and the reasons why you stayed in the background.
Thats the only thing I can think of is start there and then move out. But as a friend and one who has done so much for him, I would think he would be willing to help you out.
I hope it all turns out for you, best of luck! Please keep us updated.