I think it would help to make some kind of temporary shelter for the remaining baby. There should be no reasons for mom not to return, but what she would do after - is a question. Maybe she would make another hole and take her baby there? Or she would think the area is too dangerous and just abandon it? I know babies who are somewhat older could follow the mom because she has milk... maybe she would take the baby some distance away and hide him there in a new place?
How large is the babe? any fur?
How dangerous is this area in general (about predators) ?
another advice is to contact wild rehab and ask what to do.