It's a shame she wasn't made to wait, it just reinforces for her that she can be a bully and get away with it.
I really dislike line cutting, to the point where I embarrassed myself recently. A clothing store had a big sale on, and I was waiting for what seemed like ages in the cashiers line - there were two attendants, though only one actually taking payments and they were incredibly slow, well this lady came and stood to the side and only very slightly behind me. She didn't speak to anyone else and I said to her 'oh, the end of the line is over there' (it was quite long by this stage), she replied 'I'm here with my friend' (who was behind me). Ooops, my big mouth. I wasn't rude when I said it, but I still felt like a jerk. I will be more careful next time
I can't recall if she had her own purchases or not - and I do think just because your friend is in line, doesn't mean you get to join them at their spot in the queue with your own purchases - have seen this one happen a lot.
I do think it is different under different circumstances though, for example we were waiting (4 of us) to check in at an airport and my DH received a phone call that he had to take, so he left the queue and by the time he got back several people had joined the queue behind us and we all needed to check in together, but at least did the right thing and spoke to the people waiting to check they were ok with it. Like the guy behind us said, 'sure, we're all going on the same plane, no worries'.