Hi Mohamed,
Your concern in this case is valid. If possible, educate your client that it is not advisable to do any changes directly on Production be it house banks or any other changes. Proper testing and review analysis is always required because any direct change in the Production can be hazardous.
But yes, if there is no Go on this, then your client can use the SAP Authorization concept. Give the authorization only to few very responsible people preferably at a higher level. Common access should be restricted.
Also as Anton and Mukhtar mentioned if you give FI12 to say an IT or to a super user not involved in APP business process then risk of fraud is low.
Thanks
Seema