I still say not to do it.

I understand the divorce situation, having been divorced myself (but fortunately without the acrimony I've seen in other situations). One is tempted to do unethical things for revenge. Understanding the motivation for an unethical act, perhaps even sympathizing with it, does not make the act ethical.

Getting at the miles also involves claiming to be someone you're not. That's unethical any way you look at it, especially when it's done only for personal gain and/or revenge.

In another recent thread, which some people reading this may have also seen, I posted a minority opinion that it's OK to take opened (not still sealed, therefore heading for the trash as soon as the cabin cleaning crew arrives) amenity kits that have been left behind by others who deplaned previously if one plans to use or donate their contents. Why is this case, IMHO, different? Because that one involves a physical resource, of which our dear planet has a finite quantity. Using something that would otherwise go into a landfill is a positive contribution. Airline miles are not a physical object, did not use resources, and will not create more trash if wasted. And taking an unsealed amenity kit doesn't involve impersonating anyone.