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Pretty sure that he used the correct key.
Do they need to login to the site first, to be able to use the app?

No, they do not need to log in the website
The user needs to use their username and posting key to be able to post.
Make sure that the key is the posting (there are too many keys to choose from) and that it is correct.
In my case, I had to manually type the key, because copying and pasting it would probably take a space somewhere .. If you want please tell him to talk to me on discord or to check below :

.1. How to find the Private Posting Key

You visit the link >> https://steemit.com/@myusernamehere/permissions
You click the 'show private key' and you copy-paste that key to actifit field

privatekey.png

Let me know how it goes :)

Probably was the posting key problem. My bad :)