With time, I realized that some people are entitled by nature, and that somehow they always find a way to make every situation about themselves. If you're lucky enough, these people won't be in your life, but if you end up understanding that your mother-in-law is one of them, you should try to keep her as far as possible from you.
There's always a reason to have a baby shower and to celebrate this big arrival, but there's no need to have one if the future mother is not into it. It's normal; not everyone wants to have one, and there's no big deal. When this wife told her mother-in-law this, the woman went crazy because she didn't understand her motives behind this call. That's why she suggested she could organize it for her, and the wife accepted, since it seemed like a kind offer.
When time went by, and the celebration was about to happen, the guest list became an issue. When the soon-to-be parents read the names, they realized it only contained the woman's friends, and most of them weren't familiar to her son. At their baby shower, they'd have a bunch of strangers congratulating them on their baby. They realized that her mother was throwing a party, and they were only there to pay the bill.
They started to delete some names, mainly of people they'd never met before. The mother-in-law got very upset and told them that keeping that same guest list was the only condition for why she decided to organize the event. If they wanted to change it, she wouldn't be a part of it. Imagine having to hear that? At the end of the day, who's the one carrying the child? Who's the one who should be celebrated? I'm sure it's not the mother-in-law…
Now, the couple is about to decide what to do with the event. They either cancel the whole thing and stick to the original plan of not having a baby shower, or roll with the punches and just make the woman happy.