Not quite sure why you need a boundary with a friend, why not simply say; "we have already discussed this subject, and we obviously disagree." Then change the subject.
As far as the AH, I'm not sure you need to "communicate" anything additional to him. You have already done this, and nothing has changed.
Saying this without malice, and with kindness, but.......
Everything has a price, and until you are willing to compromise/ change your standards/expectations you will continue on the merry-go-round from hell. I have been reading your posts for quite sometime now, and I feel as if I know you well enough to say this. Please know I am not attacking you, I really feel for you. I know you have your son's best interest at heart, but you have to take care of you FIRST, so you can continue to nurture your son.
I have a feeling if you could continue to live in your current lifestyle without any change of financial status, you would already have left your AH. So, IMO that is your answer. This is now about money. And everytime I have ever based a decision on money, I paid the price.. HUGS