Don't get me wrong, I do worry about all of this. I'm praying that we can and will use him as a teaching tool for many things- good and bad. We don't just sweep it under the rug, we talk about it - A LOT. He knows why our friend isn't here and that he needed to get on with his life. I'm a good mom and my husband is a good dad and I'm sure we've made mistakes, but I'm trying hard to be aware of them and deal with them as they come up.
The difference bw your son learning healthy things from the mistakes you and your H make and those that your friend makes is that you and your H are aware, conscious, in tune with and learn from your mistakes and take active steps to not repeat the same patters. Your friend, as a non-recovering addict may know that he's made mistakes but doesn't seem to be demonstrating any desire to learn from and actively change those mistakes and not keep repeating them.
I worry about your son seeing not just unhealthy behavior from your friend, but seeing how co-dependent you and your H are (even though your heart is the right place and you have the right reasons for wanting to help) and growing up and getting into relationships himself with people who suck the life out of him.
Read the stories of those of us with spouses or parents with addiction issues... I'd wager to say that most of us grew up with unhealthy models of behavior from our parents and even though you are a loving mom and your H a loving dad, your willingness to put your welfare second to your friends' addiction is teaching your son to put others before himself.
I grew up with a fairly religious family (my father had been a priest before marrying my mother, my mother was raised in a strict religious family) and I grew up believing (despite what went on in my own home) that loving and good and kind people put others first and that you felt joy when you gave instead of taking. And to a certain extent this is true. But taken to an extreme and feeling (as I have my whole life) that ANY need I had, any desire to take care of myself made me a selfish person has set me up to be a very unhealthy person.
You want your son to be a caring and kind person as you and your H clearly are but not to the exclusion of being able to care for himself first and see that sometimes love means having to set a boundary and hope that the person you care for takes the opportunity to examine their life and do what they need to. All that will happen if you continue on the path you are on is that you will become increasingly upset, it will probably spill over into problems with your own H and family and your son will grow up thinking that the needs of those who are ill and who have no interest in helping themselves, take precedence over the needs of everyone else.
I'm making some really tough decisions in my own life right now not bc they are comfortable or bc I really want to but bc I need to for my D's sake (and mine). I don't want to deal with the anger I know will come from my AH as a result of my choices and the status quo is, in a lot of ways, easier in the short term. But I want my D's to grow up healthier than I did and I want to still have time in this life to be happy and healthy and being with my AH right now isn't allowing for that.
It's okay to feel upset about setting a boundary with your friend and yes, he will probably manipulate, guilt trip, be upset etc... Expect it and that will take away some of the power of it all.
Last thought- I promise-- think about the man you hope your son will become based on the lessons you and your H are teaching him and the values you try to instill in him and ask yourself if having your friend in your life and your sons is compatible with what the kind of man you want your son to be? Sounds to me like your friend demonstrates abusive behavior toward you and intimidates you into apologizing for HIS being upset. Your son shouldn't have to see his mother treated that way- ever.