Aw yea I think it's a momentary issue hopefully. It's so hard to get a new sibling in the mix. I'd definitely speak to him and see if he can add in an extra day of time to spend just them two here and there because it's obviously affecting her and she needs that reassurance for a while
My son is still going through this now. We believe he feels like he is not good enough and therefore got replaced because of it and he has all these mixed emotions about it which he can't quite communicate well yet and that resulted in some tantrums and wanting to be babied. I don't think there is anything we as parents can do other than reassuring him he's still very much loved and spending more one on one time with him when his brother is sleeping which he absolutely loves but then seems to default back to being down and upset when he's awake and has shared attention again but also seems to be getting better now that he is able to play with his brother more. It's heartbreaking but we continue to reassure him til he believes it.
Talk to her dad and tell him that he needs to spend some one on one time with her again- even if he just takes her to the park after dinner for half an hour one evening a week. He needs to be aware that she is missing him