If she's stressed out with being caught and confined, she may be too scared to move. Normally with feral mums and babies, I part cover the cage so they have some privacy.......some cats get extremely protective of their young. Is she in a secluded spot where she can be quiet and left alone?
Food wise, I would check that he's giving her enough food, they tend to eat 4 times more than normal when feeding young. Make sure it's a good quality food and it may be an idea to get some kmr for her to keep her strength up. If she seems obviously weak, a trip to the vet needs to be done but ideally, if he can explain the symptoms and hopefully the vet will give him something to try her on.....she may have an infection but the vet isn't going to be able check her anyway and it will totally stress her out taking her.........she may need some antibiotics by the sounds of things.