Create a custom admin panel

Your question is pretty broad but provide your client with a custom role and only allow access to the specific pages you want, whether those are Core or created by yourself.

That is how I would approach this. You will be working within WordPress and so retain its functionality for yourself but have a truncated interface for your client.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)