Is it just me or does it seem we have gotten way off base on this scenario?
At this point, from all of the OP's posts, I do not get anything malicious behind her boss and co-workers, just plain cluelessness. They are under the assumption that the op does not like going to events like this.
The op admits that she does not see graduate all that often, so I can see graduate asking co-workers, especially if a young graduate, (is this high school or college?), if she should invite op or not.
Boss and co-workers sound clueless. No, she probably would not attend, but I am sure she would give a gift. Not that it matters, but what is the dynamic of these co-workers? Male or female?
OP, I still think you should approach grad yourself. You are not fishing for an invite. It was stated to you that graduate did ask if they should invite you. It sounds like graduate was going to invite you and I don't think you were meant to be excluded. Tell graduate there seems to be some miscommuncation.
Maybe don't even approach gradaute, but next time she is around or your co-workers start talking about graduation party, this is where you need to step in and state your mind, calmly. Your opening was when graduate said that they should not be talking about this in front of you. "You know, I would have gladly attended said party if I had been invited. As it so happens, I now have plans that day. (because at this time, no matter what, I don't think I would be attending). Co-workers, it is not up to you to make decisions for me. Next time, please let me decide for myself on what invitations I will or will not accept".
I am saying this because it sounds as if you are hurt by being the only one not invited and from what I am gathering from your posts, it was not the graduates attempt to do this.