Was I unfair to my co-worker?

I was recently tangentially involved in a debacle at work, mainly concerning another project team I used to be a part of (and still collaborate with), and need some guidance.

Basically, someone at my company, who I’ll call M, made a serious error, stemming mostly from incompetence. I don’t work on M’s team but our teams do collaborate, so I was one of the first outside his team to notice the error, which I decided needed to be brought to the attention of the manager of both groups. Before doing so, I reached out to one of M’s team-mates, J–who I know pretty well and used to work with pretty closely–as I was curious for his insight. We both then met with the manager, who agreed it was a big problem–one that J and M’s group needed to take to a higher level manager.

After several meetings, J reported back that the managers decided that he and a couple other team-members also bore some small amount of responsibility for what happened. Not as much as M, who had done numerous things he shouldn’t have, but still, the managers thought, J and a couple others should have caught what was going on.

J then told me he was really upset by this: it was M’s incompetence that caused the issue, and anything J himself did wouldn’t have been a problem if M had known what he was doing. I told J that I would stand behind him, if needed, as its true enough that everything he did would have been fine if M had understood the rules. There were places where the team didn’t fully follow procedure, but that often happens within our teams, as we’ve always been able to trust that people were doing things right. (And if we’re talking about the responsibilities of people other than M, the manager should absorb some, too, given how lax he’s been about procedures…..)

That said, I also do actually agree, to a point, that J could have caught and stopped the issue from developing much earlier. M’s incompetence was fairly clear all along, and J let M do some of the things he did because it took work off of J’s own plate. I absolutely won’t tell the manager this, as he would absolutely take it to an unnecessary extreme, and I don’t think J should be, like, officially reprimanded.

But I guess I’m also, more personally, a bit frustrated with J, because he won’t even acknowledge that he could have done something differently. The whole situation caused a massive headache and days (at least!) of lost time for everyone involved, and I feel like if we don’t acknowledge broader problems, I think its likely something like this will happen again…..

But J is also a good work friend and I don’t want him to think I’m throwing him under the bus by pointing out those broader problems…. I’ve tried to talk a bit about this with some other co-workers, but they had mixed reactions, and I guess I’m just left feeling bad about it on a personal level. Mostly I worry that J will think I’m being two-faced: saying I would support him, while also feeling like he could have done something differently. Does this make any sense? Does anyone have any advice about how to proceed with this, either personally with J or in terms of the broader work issues?