This error is commonly caused by something going wrong in the Team Create server side. Is your friend able to enter other Team Create places?
In addition, servers will commonly reset after 10-15 minutes of no connection, so I would recommend waiting a while and seeing if you can reconnect after waiting. If this problem persists, seeing this user’s logs would be appreciated (on windows these are found in %localappdata%/roblox/logs, send the ones modified when this problem was occurring). PMing me those would be immensely useful.
Also, we don’t recommend turning off your firewall. If a connection to Studio was working previously, a firewall is likely not what is causing the problem.
I agree that this sounds like an application issue. If you could PM me the logs if this problem still occurs, that would go a long way to helping us resolve this issue.