Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
customer [ˈkʌstəmər] n (of shop) покупа́тель*ница) m(f) ; (of small business) клие́нт; (of large company) зака́зчик
he's an awkward customer (inf) он тру́дный тип
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: