Underrepresented minorities such as African American, American Indian, and Latinos are the ones that most benefit from their status. While this person can mark Arab or Other on the transcript, they will almost assuredly be considered with the white, asian, and other non-minority applicants. In other words, that person is not considered an underrepresented minority by most dental schools.
The reason is that this country has a far greater need for african american, indian, and latino dentists in those respective communities. During the application process, there is an effective difference between being a minority and an underrepresented minority. Make sure the Lebanese applicant realizes the differences between the two or he may be in for a rude awakening when he is does not receive the treatment he was expecting during the application process.