The PLAY:5 gen 1 will only work with the S1 operating system and controller. It can not be added to an existing S2 system.

You could set it up as a parallel system, then use whichever appropriate controller, depending on what Sonos system you’re using. There is no reason why both the S1 app and the S2 app can’t both live on your phone/tablet/desktop machines. 

Yes, I have it set up on a parallel S1 system that has only the Play 5 Gen 1, the other Sonos speakers I have are on the S2 app. The issue is that I have the Play 5 Gen 1 working with S1 on one phone, but I can’t add it to the other.

I’m confused by your actions. You don’t pair a speaker to a phone; the phone is just a controller for a system. If you set up an S1 system with your phone and added the Play:5 to that system, all you need do with her phone is install S1 app and access the system containing that Play:5.

The Play 5 Gen 1 is working with the S1 app on my phone, I cannot get it added to the S1 app on my wife’s phone. My other Sonos speakers are all workign with the S2 app on both phones.

You don’t add it to the app on her phone. As I already said, she needs to use her phone to access the system you’ve already created. The speaker can’t exist on two systems. 

I completely understand this. I’ve had Sonos products for 14 years and I’m very techy. I reset the S1 app on her phone and it will not connect to the the new Sonos system I setup on my phone. 

So, then, describe your WiFi network, please. The fact that your wife’s ’remote control’ can’t contact the existing system suggests that it is connected to a different WiFi or subnet that the Sonos isn’t on, but your phone is. Any chance you have some sort of mesh network?

Have you called Sonos Support directly to discuss it?

Then you should understand that to talk about “pairing” and “adding to the app” is very misleading. 
 

Does the app have the necessary permissions? No corporate software halting things?

I have a Ring Eero router with extenders that do create a mesh network on the upper floors in my 4 floor townhome. All of the Sonos speakers are in the same IP space, 192.168.x.xx. I have only one network setup with a single SSID set up for the home and we are both connected to it. As I’ve said, the newer Sonos speakers are all on the S2 app (2 playbars and a newer Play 5) and work on both iPhones with no issues. The Play Gen 1 is on a seperate system with the S1 “controller” app. When setting up the Play 5 Gen 1 I brought it down to the ground foor next to the router. I haven’t called Sonos support yet, but I think I will.