I have always believed that we should not really interfere with our children's relationships - its more about standing back and offering a shoulder to lean/cry on, than having one's 'five penneth worth'. However I do think I take issue with what your husband appears to advocate. You say that he feels you should 'support him no matter what his decision is'. From what you say he has already made some pretty dire decisions and I am not sure support is what should be offered. From what you say he sounds a very shallow person who leaves others to pick up his mess.
I think that I would seriously consider finding a tactful way of letting this young woman know that although you respect their privacy and do not wish to be directly involved, you do want her to know that you are thinking of her and that you would like to be considered her friend.
Many years ago my then Mother in Law stood back and let her son walk away from his pregnant wife (me) and make what he later admitted to be, the worse mistake of his life. She lost a grandchild as well as putting distance between herself and her son as he found it very difficult to live with the guilt of what he had done. Her 'not getting involved' allowed him to only see his side of the situation at a time when he needed someone to make him see what he was really doing.
That being said, try to separate the sadness you feel from this situation. Your emotional response to this is not something they should have any control over, that's for you to come to terms with whatever the outcome. Your happiness should not be reliant on how others live their lives. You only have control over your life, and even then its only for what is happening now and not what has passed. Good luck and just try to stay calm about it.....knee jerk reactions to these types of situations are never wise.