I havent messed around with this kinda stuff in a while but there are several ways to do it.

Is it an Iphone and they are still running a shared Apple ID? That is the easiest way to do it and si cr she has done a hard reset this is the most likely. My wife and I share an Apple ID and Apple update every once in a while will set my master phone to where her calls ring through to my phone. Can do this from a computer as well.

Samsung / droids can work same way but more difficult to do with shared access.

You can jailbreak a phone to do it but at 54 years of age he would need help unless he is into phones / IT for a living and doing it this way is illegal.