This is such a sensitive topic, sleep takes on a whole new meaning as a mother and our empathy meter goes through the roof. It's hard to hear about this because we look at our own babies and can't envision doing the same with them.
That being said, I think I personally would have taken a different approach. I've been in a similar situation, and I'm definitely a minority in my local community. Nearly everyone I know has done CIO. One did it with both children from 3 and 7 weeks, and I won't even go into the other dimensions of their parent-child relationship that are distressing to me. Instead of feeling like I have to prove what the other person is doing is wrong and letting the situation get combustive, I try to share my personal experience and say "this is what worked for us, this is what didn't work for us" and leave it at that. I'm beyond the point of trying to understand why people choose what they do with their children, but if they really want a different way the information is out there for them to find without my help.
Now if someone were to come to me and say "help me with the sleep issues I'm having with my baby", I'd offer suggestions to them. Otherwise I try to focus on my own family, it gets exhausting when I spend too much time being concerned about what someone else is doing.