You were not there in the moment when Ted put his arm around her. Did he do anything more than a friendly arm? Did she protest at all? Was she obviously uncomfortable with that in the group? (Of course, I would hope that people would have enough of a friendship or aquaintance-ship to have established boundaries for touching/no touching, but some people...) If you had been there, it would likely have been easier to navigate the situation and tell him to lay off in a lighthearted way with less social pressure. You would have had all of the knowledge from the questions posed above to know how to act whether it was giving him a passing "that's not cool, man," calling him out, or simply removing you and her from the situation.

I think you did well in waiting a moment and telling Ted more privately that his actions were not acceptable. Ted should have also apologized to the girl (under your supervision but out of earshot so that she feels safe). You could have helped facilitate that apology and been like, "I was wondering if you could apologize to her? She is a bit upset, and maybe that would help," but I understand if you didn't think of that in the moment. That would probably have helped confirm for her that you did in fact talk to him, he completely understands, and he won't do it again.

You are not just as bad as the people committing the acts. If you had seen the situation happening in front of you, I believe you would have had a better chance to demonstrate your belief in standing up for women.

However, I understand her point of view. She might not be interested in you anymore, so whatever happens happens. If she wants to base her decision to end the relationship on a very "gray" event, she can, and that's okay. Maybe what happened was more black and white to her. At this time, it looks like y'all are having trouble seeing each others' viewpoints.

Some situations can be diffused with a lighthearted "cut it out" or "that's not cool, man," and others should be taken more seriously with in depth conversations or even a direct confrontation. It all depends on the social context, and you didn't have that context. Regardless, you did your best, and hopefully you are better equipped to handle similar situations in the future if you can take it as a learning experience. Nobody is perfect! All we can do is try to be better!