Based upon your post, I would agree with you. I don't mind my friends jumping in at the last minute to get rewards when I've soloed the Nightfall as I've been invited into groups just as they were finishing raids. This though is not acceptable behaviour and I don't know anyone who would have allowed me as a friend do this, let alone a stranger. It sounds like you gave every opportunity to join and they didn't. Definitely their fault it ended this way.