Is the boss’s request a form of racial discrimination? Probably. Language usage, accent, and style are closely associated with race and ethnicity.
You must make your hiring decisions without considering the candidate’s accent or diction. Your boss’s request is inappropriate and discriminatory.
Is your boss’s request valid? Will your clients expect standard speech and think less of your company if you hire receptionists that they perceive to be uneducated? If so, a person who uses non-standard English may not be as successful in the position. The position likely requires excellent communication skills and the ability to speak in a way that can be easily understood by coworkers and customers, and these requirements should be clearly defined in the position's job description.
Care must be taken not to let a candidate's accent or diction, or any other factor that may be related to race and/or ethnicity, influence your hiring decision.
After you interview and select the most qualified candidate for the position, regardless of his or her speech patterns, you can coach the newly-hired employee to use standard English and proper grammar when speaking with clients.