Re: How would you have handled it?
The being seen as a outsider is the bigger issue. If she still hasn’t gained that kind of respect and trust then she needs to figure out how. If the house is new and the change of furnishing is immediately after mil makes changes then it’s practically a new bahu issue all over again. It’s not PC to change curtains after a month or something.
I think wife should have taken her husband into confidence beforehand because it’s his room too. This would have given her an idea of what to expect instead of letting him see the finished item or telling him dude I’m making changes unilaterally. Some tact is required to coax her husband to see the changes are merely because they look cute and not an attack at his mother (although anyone who reaches that conclusion first is kinda weird already). If the husband still doesn’t see how dumb his stance is then it would be time to time to stop playing coy and flat out having a discussion about how she feels like an outsider.
I’m not saying she’s an outsider. I’m just looking at the situation and seeing it as that. It was not after a month but 1 or 2 yrs later that a change in how it looks was considered if I remember.