There is a very important piece of information that has been omitted from this question: how old is their child?
If she is still breast feeding, if the child is not sleeping through the night, if her hormones are still out of whack this situation is likely to resolve itself on its own IF he is patient and supportive and accepts that having a child DOES change things.
However, if their child is older there is more at play. She needs to see a doctor. She needs to talk to that doctor about her low sex drive.
As far as this "she's a feminist and that's why she doesn't want him seeing her naked" nonsense, it's exactly that. It's nonsense. That has nothing to do with her sudden body shyness.
My guess is that she has created a negative feedback loop for herself. She feels guilty that they are not being intimate so she feels uncomfortable when he sees her naked. When he apologizes she feels guilty that she feels uncomfortable. So she wants to avoid him seeing her naked.
Again though, the age of their kid indicates what he should do about it. If their child is young (say, under three or so?), he needs to take the pressure off of her. Give her the chance to enjoy physical intimacy with him without the expectation of sex. That means no whining, no guilt tripping, etc. Sex is off the table until she is ready. He needs to tell her that and he needs to mean it. Performance anxiety is not a sexy feeling.
If their child is older and this has been going on for a while, he needs to ask her to see a sex positive couple's counselor either with him or alone. And again, she needs to see an M.D. as well to get checked out. She might have a hormone imbalance like low testosterone or there might be any number of other things going on.
Regardless of the conversation he has with her, he needs to have a conversation. He needs to be calm and non-accusatory and he needs to be willing to listen. And if they work on it together for a good long while and there doesn't seem to be a fix but he wants to stay in the marriage, he needs to talk to her about opening it up sexually.