Girlfriend's jealousy might be justified
You seem to believe that this is fine, but I believe that her behavior was manipulative (toward you) and unethical (toward her boyfriend).
If this is the only one of your female friends your girlfriend reacts negatively toward, then it would be respectful of you to try to see this from her point of view.
You could be helpful regarding her insecurity if you continue to reassure her, and also offer to include her in your social interactions with this person. Where is this woman's boyfriend? Could you ask the two of them to have drinks with the two of you?
Dear Amy: I recently went to the movies with my daughter. It was stadium-style seating, where you pick assigned seats. The young couple to our immediate right was talking throughout the previews. Not a problem.
They continued talking when the movie started.
I waited a few minutes, then leaned over and politely asked them to "please stop talking during the movie," to which the young man replied, "No."
They then continued to whisper during the movie.
What else could I have done? To find an usher, I would have had to climb across the row, past 10 other people.
The only other available seats were in the first three rows.
It was a distraction for the entire movie. Outside of raising my voice to tell them to stop (so others could hear), what should I have done?