Wow. I must say i have to check my privilege at the door here. I did not think about this use case. You are absolutely right. If you only have a phone and access to the app you can no longer submit to a pub!!!
Sometimes when I travel and I need access to the publication instead of going to the app i go to the mobile website. Then i request the desktop version and with that I can access the publication. Perhaps you friend can try this work around?
I am not suggesting that in anyway this makes it ok that these features are not available from the app. They 100% should be. But it may allow him to still publish with you publication.