Assuming you tried going through whatsup first, report them to police or cell operator, let them deal with it: they might have access to which actual phones (IMEI) have been used for those numbers, and if those devices have also been used with non-burner numbers.

If they are indeed multiple burner phones (different IMEI numbers), then police it is: they can get IP and similar data from whatsup, etc.

Remember, you don't need to know who is harrasing her, just for them to stop!