Ok, so I managed to find half a solution for you :)
If, as a player, you go to the main, app.roll20.net and act like you're a GM who forgot their password, you'll be emailed a recovery link, which, if you follow, will log you in, and take you to a landing page that essentially tells you "you're not a GM, you shall not pass!"
Now, at this point, you're actually logged into your account. So, without closing the window, paste in the campaign link, and join your campaign.
I searched high and low, and you are correct. I could not find any way to do anything to my account as a player.
But at least this way he can play in the meantime.