I may be playing pop psychologist a bit here but I was reading a book I bought today about why people who grow up in dysfunctional/abusive homes tend to repeat those patterns in their marriage and here's what popped into my head when I read your post (bc it's what I was reading today)...
I'm paraphrasing... A lot of us who grow up without (without love, without stability, without safety, without sobriety in our caretakers) etc... get involved in relationships (in my case my marriage, in your case your friend perhaps) where we try to recreate what we did not get as kids. We want to "fix" what we couldn't as kids. So, I grew up with an abusive, emotionally cruel, abandoning, verbally abusive mother. And I married a man just like her and have believed for a long time that he was a tortured soul/misunderstood and needed love and compassion and I could help him get better. And I read your post and wondered if maybe there isn't some similar sort of savior complex (I don't mean this obnoxiously) that you're feeling for this friend bc you couldn't "save" your dad, grandparents etc... as a kid...
I'll apologize in advance if I am drawing conclusions about a situation I know nothing about-- it just occurred to me that what I read really fit me and it sounded a lot like what you describe too.
For what it's worth- my AH means the world to me and to my daughters. He can be a loving and good man at times and he is the father of my children and for that I love him. But someone meaning the world to us doesn't mean that we sacrifice our own peace/life/happiness/well being/sanity in order to try and help them. I'm slowly figuring that out.