"There are no medical reasons for this. Baby is three weeks old"
They've had a baby smack bang in the middle of cold/flu/ covid/rsv season. That IS a medical reason. Have you ever seen a baby with RSV? it's horrendous to watch. I gave birth to ds the same time last year and we limited the amount of visitors to give him a chance to build immunity slowly and reduce the risk of him picking up something awful while so tiny. I have other friends who had babies at similar times who ended up hospitalised due to rsv, covid and really bad respiratory infections while they were still so little.
Your need to cuddle an adorable baby doesn't outweigh the mothers right to make what she feels is the best decision she can to protect her child, especially at a time when people are socialising in big groups and babies are often passed around like parcels.
She may be really struggling with not being able to breastfeed. Of course there's absolutely nothing wrong with formula feeding, but if she really wanted to breastfeed that can be really upsetting and many women feel genuine grief over not being able to do that.. the lack of support is the reason why we've one of the lowest rates of bf in the world. She's not wrong about that and she's every right to be angry if she didn't get the support she deserved to help her feed her baby. She's absolutely entitled to those feelings.
The other thing I would say is that women who struggle with fertility and eventually go on to have a baby often are at higher risk for ppa and ppd because they have maybe very big expectations for themselves, a lot of anxiety through the pregnancy and its very difficult to talk about what you're finding hard when you feel you should just be grateful and not complaining that things worked out, you're actually furthering that narrative through your talk of her journey with endometriosis in that way. That in itself is a risk factor for her emotional wellbeing.
There are also some studies that encourage parents to keep things small for baby in the 4th trimester. Baby has had a massive adjustment coming into the world and there is so much to be stimulated by and to process. It sounds to me like she is doing what she can to help baby make that transition gently. Hearing their fathers voice, mothers voice and smell and skin to skin is proven to soothe babies from the get go. What she's doing is very direct but I really admire her for setting her boundaries and thinking about what she and baby need first and foremost because really they're what matters right now. If more mothers did this, I think more women would have a better pp experience because they'd be less likely to need to deal with pushy relatives prioritising themselves over the new mother and baby. A lot can change in a short time as you've recognised, it doesn't mean you did things wrong - you worked off the information you had available and the recommendations at the time. She is doing exactly the same. Support her choices.