Can I do the Recruit-a-Friend campaign under these conditions?

My brother bought the game and used up the free trial. It's currently inactive, but he never subscribed. Can we still use the friend recruitment codes to start up his subscription?

I've seen TONS of posts saying you can do it DURING the 30-day trial, but I've seen ZERO posts about doing it AFTER the trial, before paying for a subscription.

Also, SE, please add a FAQ for friend recruitment. Thank you.

As long as he hasn't subscribed you should be fine.

^ is correct, as long as no subscription has been paid for, you can redeem the code
the recruitment code does not add any time to the account, though

Why didn't you simply try it?
I mean the worst that could have happened is that your brother got an error message on the mogstation.