Here's what he should do. Get Google Voice. It will put both of his numbers on one phone, with visual voicemail if he wants, and everything will work seamlessly and automatically, without having to do anything when making or receiving calls. He won't even have to think about it. You can do a lot of very cool stuff with Google Voice.
This is gonna sound complicated, but it's really not hard and the results are SO worth it. I've used the setup described below for years and thousands of calls and so has my wife. Our businesses have depended on it. We have had NO problems, totally reliable. And this does *not* use Hangouts or the Hangouts Dialer.
First, get a Google Voice number on the Google account of the phone that will be kept, the one that has a number that will stay as the regular cell number for that phone. To make the Google Voice number act as a second number on that phone, just go to GV online and in settings set it to forward all calls to cell number of the kept phone. I use my cell number as a private friends & family only number and use the Google Voice number as a business number and for everything else.
The other phone will be discontinued and the number from it ported to Google Voice. It then becomes the GV number. It's also now the second number for the kept phone. This way the numbers have not changed and no one has to be notified of a new number.
OK, so both original numbers are on one phone. Not hard - get GV number, set forward calls, port discontinued phone's number to GV. Very nice. But we're about to really take this to another level.
Install the Google Voice app and you have visual voicemail for both numbers in one place. During setup choose GV for all voicemail service, not your service provider's voicemail. This activates conditional call forwarding.
Remember, in initial GV setup outlined earlier, GV was setup to foward all calls to the GV number to the cell number? Well, now unanswered calls to the cell number are forwarded by conditional call forwarding back to GV so GV can handle all voicemail, for either number, in one place and as visual voicemail.
Now, we get to what, for me, is maybe the best of all. To really make GV work seamlessly, if you haven't already, organize your contacts into groups - Business, Family etc. Then install VoiceChoice 2.0.
It works like this: Setup VoiceChoice to, for example as I do, call all Business Group contacts using the GV number and call all Friends & Family Group contacts with the real cell number. Set the GV app to make all calls through GV by default.
But VoiceChoice 2.0 actually dials out. It automatically chooses to use either the cell number or GV number, depending on the contact group the contact being called is in. So, Business contacts see the GV (aka business) number, and Friends & Family contacts see the cell number.
This works perfectly. Routing is automatic. You don't have to do anything special or remember anything, just use the phone normally and GV and VoiceChoice 2.0 automatically deals with which phone number is used for outgoing calls. With VoiceChoice you can also choose which number is used by individual contacts instead of by group.
And GV wil do specialized voicemail messages by contact group. Business customers hear a business message, family a family message,etc. GV does all this and much more.
Anyone with questions about this please feel free to ask me. I know it seems like it would be a hot mess to setup, but it's actually pretty easy.
https://play.google.com/store/apps/details?id=com.google.android.apps.googlevoice
https://play.google.com/store/apps/details?id=com.voiceplus