It’s the s2 app. I remember when the previous apps gave you the option of not now etc.
Thag page just has update and nothing in the more options allows you to not update and keep going past
Is it a newly downloaded S2 App, or one that was used previously with another Sonos system? - if the latter, then I would remove it (just for ease) and download the S2 App again from the App Store.
I’m not quite sure if it’s the latest version of the S2 App, as I think the new version has a slightly different screen, but it maybe because I’m using an iPad controller here.
If no luck with a fresh install of the S2 App, then perhaps try the S1 App instead… it should find the system and then you can just decline to do the firmware update… if problem still persists after that, then I would call the owner of the system, as mentioned earlier, to perhaps get the devices updated.