I also think one of your options would be to drop a brief note to this woman. Be polite -and the great thing is that you get to totally choose what to say. I would think a carefully-worded message would get you off the hook, ease any guilty conscience, and communicate that further communication is not likely to happen.
I being me, would also probably try the "call when you suspect no one will answer" tactic. I personally would not really want to not return the call, nor would I want to engage in a discussion about my son.
I know that my first-cousin has fallen off the wagon numerous times. I happen to have a good relationship with his father, however, when we see each other and are catching up, all he needs to say is "Brent isn't doing too well right now" and no details need be disclosed.
Just an idea.