While not technically "self" defense, it may that he was legally acting "in defense of others." It would be very fact-specific, but a person can generally act to defend others as well as themselves, assuming their actions were reasonable and did not go beyond what was necessary to address the imminent threat.
Answers given are general in nature and not intended to create an attorney-client relationship. Specific advice can always be obtained by contacting an attorney directly.
What is he charged with?
If he reasonably believed there was an imminent unlawful threat to his girlfriend, and she would have been entitled to use similar force in her own defense, and if he reasonably believed that the force he used or threatened to use was necessary to protect her, he may have a defense to a charge based on the act of pulling the gun to protect her. If the defense is factually in play, the State will need to prove beyond a reasonable doubt that he was not lawfully acting in her defense.
If he is charged with unlawfully possessing or carrying the weapon, the legal analysis is different.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.
Self defense also includes the defense of others. I agree with Attorney Witt. If he reasonalby believed that deadly force was necessary, he could assert "defense of others" as an affirmative defense.