Dear Member,
Grammatically, your first sentence is correct since the indefinite article expresses the concept of singularity.
What you have encountered here is not a gramatical problem, but rather a cultural problem now that a shift towards gender equality strives to become the politically correct norm.
For example, compare and contrast the following sentences:
A friend of yours wants to study in the same school you did. Give him or her some advice about that school.
A friend of yours wants to study in the same school you did. Give him/ her some advice about that school.
A friend of yours wants to study in the same school you did. Give them some advice about that school.
Culturally, this would be a high-context approach at solving this grammar problem since the third person plural object pronoun fails to relate gender while the animate singular forms directly express gender.
I usually teach this advanced grammar concept after correcting my students' old textbooks that still have "fireman, policeman, mailman" and the like to the new politically correct form of "fire fighter, police officer, postal worker".
Depending on the age and cognitive development of my students, my methodology changes as to how I incorporate the cultural concept of gender equality in my classroom, but all of my students know to use the correct forms, regardless of their age.
In cultures where gender discrimination is still found, this phenomenon should not occur... although hopefully, one should be able to observe its occurrence there in the near future.
Sincerely,
Eric Paul Monroe
http://www.eric-tesol.com/