Well, if it makes you feel any better, I think you're both wrong, and for the very same reason.
One isn't a "true Mormon" because he's 100% obedient to all of God's commandments as interpreted by the Church. If that were the requirement, I could not claim to be a true Mormon and neither could Thanda. The same could be said for Christians in general. When it gets right down to it, the same could be said for the adherents to the various political parties, hence, the emergence of terms such as "RINO." If a person isn't 100% on board with what someone else thinks believes Republicans must stand for, he's a Republican-in-name-only. That's pretty lame, IMO. We all have differences of opinion and we all have different standards. Some Mormons might say I'm not a "true Mormon" because I am such a strong supporter of LGBT rights. My former LDS bishop (men only serve as bishop for five years, and he is not my current bishop), however, would disagree with them as he was fully supportive of my position.