Have you checked with your IT team to see if they updated anything on the backend? It’s odd that it’s only one user in your instance? Is your colleague a ‘user’ or a ‘guest’ in your account?
What I find strange is the way this is showing up on the screen.
As far as I know there is no option on the Admin Account to block only the Marketplace to non-owners, either members or just guests. I opened a ticket with our IT but still waiting for their reply.