think about this, if you were him/her would you hire someone to have access to your BTC funds? do you think his/her reason is valid or enough i youwere him/her?

Anyways I am not that familiar with the website, maybe there is an option there where you can sign in as a trader for someone and that you can not withdraw his/her funds and maybe that is the reason why he/she is confident that a guy like you can not transfer funds away from his/her account.