Try separating them but so they can still see each other and see how she is,some queens will accept another cat present at birth but because their natural instincts usually kick in they like to be somewhere quiet and private with their "midwife-you",also though he is affectionate with her,he may be different with kitts around, he could become jealous that her attention will be on her babies and react accordingly
Good luck and hope all goes well for her and her kitts