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!
One potential solution.Assuming you are in US , get a Google Voice number and change your wife WhatsApp account to that number (Account - Change number). This way she would not lose all her chat history and continue use new number which the person won't know. You route calls from old number in Google voice. So everything will be usual just the person harassing will not have her new number
how about spaming the harraser with whatsapp bots? If you are using iOS you can do it without a library or service just use siri shortcuts to make a spammer. like send 100 messages for every recieved message here is a easy to use spammer i found on github (not tried) https://github.com/gaultierlecaillon/whatsapp_spammer edit: added link to github repo
Change phone number and open new WhatsApp account.
Blocking them as they come up or changing accounts seems easiest.
WhatsApp?
You can send them an image payload with spyware embedded in it
Forget technology solutions. Depends on who the harasser is. I would respond to them with something like "You have been reported to the police and feel free to keep doing this". I would still report this to the police even though I know they won't do much until it becomes a real threat but a record with police is a good start.