Are you sure the user is able to connect to the SQL server from their machine? Perhaps the connection is blocked, the hostname is wrong, the ip is wrong etc.
The user is blocked from connecting to the sqlserver, so we will add ip to the firewall rule and make exceptions until we hit a block of users who needs access to sqlserver.
It sounds like this will solve your issue then.
If so, please mark the answer as correct to close out the thread.