Apparently, this is an ancient behavior; mothers-in-law have been getting too involved in their son's relationships for a long time. The only relationships that survive are the ones that manage to keep the mothers-in-law at bay.
I'm not kidding; I've seen this play out in my family and outside, when I was a daughter-in-law. I guess mothers can't help it; their daughters-in-law spark some kind of subconscious competition inside of them.
I guess it's because they want the best for their children, and they are wired to protect them, so when they see something they don't agree with happening in their son's relationship, they step in. Of course, if they are well-adjusted human beings and mature enough, this shouldn't happen at all. But sadly, it happens very often.
We should trust our partners enough to handle this, because that can be the only thing that protects us from their intrusion. It can be difficult because we're talking about a mother-son relationship, and its dynamics can be really hard to change, but as a baby comes, it can push those hierarchies to adapt.
If the husband can't do it, somebody else has to; there is a baby to protect now. It might take long discussions and boundary-setting practices, but it has to be done.
Parents should decide how to raise their child alone, without everyone feeling free to comment on it and then play the victim. I'm sure if she'd communicated better, she wouldn't have received such backlash. I can only hope the husband in this story sees that as well.
I've seen marriages fall apart because of a mother-in-law. Do not underestimate them. Some men become fathers but refuse to stop being children. Sadly, some women often find that out too late.