Joraemi's daughter has not done anything to AG. To the contrary, AG has intruded on her time and space, has trod upon several boundaries. By giving in and allowing AG to, once again, behave inappropriately, Joraemi's daughter will be sanctioning that behavior, and she will allow her prom to be about AG instead of about her own enjoyment.
But AG hasn't DONE anything yet. Several of us have said that if and when she does -- at the prom -- that is the time for the OP's daughter to do something about it. Not pre-emptively, because that would be presumptuous. The OP has said the pre-prom activities are not going to be a problem.
But sparing her the public humiliation of being told AT the prom that the person she most wants to spend it with - does not want her around, would be a great kindness to AG, IMHO.
Pod. In high school, DH had an interest in a fellow senior who was nice to him and considered her his friend. Their senior class went to Disneyland for Senior Night as did many high school senior classes. I know DH headed to Senior Night on the school bus with the understanding that he was going to hang out with Friend that night. They get all the way to Disneyland and, prior to getting off the bus, she tells him that she wants to hang out with her friends, not him. DH spends the evening by himself and has a horrible night. Now, DH can be a bit much at time, not picking up social cues, but he's also a sweet guy so I could see the girl not knowing how to break the news to him but really not wanting to spend the evening hanging out with him. I could see myself being that girl from his high school as I, too, had some "cling-ons" in high school (including one definite stalker who, out of frustration trying to get him to stop bothering me but still be nice and not having it work, I finally screamed at in front of a bunch of students, "Leave me the **** alone!!!"). I could understand why this girl did what she did to DH, but it would've been far kinder to let him know in advance that she didn't want to hang out with him.
I don't think it would be doing AG a kindness to wait until prom to let her know that DD and friends do not want to hang out with her. I don't know of a good way to let AG know gently (go to AG's house and tell her?). It'll suck for sure, but it'll suck more to find out at prom. I know I don't think kindly of the girl waiting until they were at Disneyland to let DH know she didn't want to hang out with him.
As far as looking back and wishing I were nicer to people in high school? I'm pleased to say I was as nice as I wanted to be in high school. I was nice to people as a rule. However, nice didn't mean I had to socialize with everyone. In fact, now that I think about it, when I was a junior and then-BFF was a senior, we and some of our friends ended up with a cling-on the first few days of school. A freshman decided to be "bold" and attach himself to this group and that group at school. It was very weird for the bunch of us to find this little kid all of a sudden hanging on the periphery of our circle. We were a bunch of goth kids so I guess he thought he'd try Goth Kid Group #1 first. We didn't tell him to leave, but we didn't encourage him because he really didn't fit in. After a few days, we saw him attempt to infiltrate Goth Kid Group #2 (much to our relief). They must not have taken to him either because a few weeks later, we finally saw him with a group that did seem to accept him. While none of us really were mean kids, we didn't want this kid hanging out with us but didn't know how to get rid of him, short of being mean (fortunately, he made the choice for us).
Another time, on Senior Ditch Day (again, I was a junior and my friend a senior -- different friend), the plan was that the senior class was meeting up at a park then going to Disneyland. There was a girl many of us really didn't like (she was super-obnoxious) and when everyone started grouping up at the park, she was one of the last individuals left without a ride when Friend and I were getting ready to leave. Friend offered to give Girl a ride to Disneyland then we must've had a mind meld (we both thought, "Oh, no, if he does that, we'll probably get stuck hanging out with her") because he then said, "Oh, I'm sorry, but I'm not going to have room in my car" so we left without her. Turns out Girl and another girl (Girl 2) got left behind (I was surprised about Girl 2 since I always thought she was well-liked -- not by her class apparently). Anyway, found out later from Girl 2 (who thought it was humiliating but laughed it off), Girl 2 decided to just go home. Girl 1, though, was peeved, went back to school and told all the teachers where everyone went (I don't think the teachers cared). Am I sorry we didn't take Girl 1 with us? Not on your life. I remember spending a fun day at Disneyland with Friend (who was one of my few friends who was able to attend my wedding a couple of months ago). I think if Girl 1, like AG, had spent more time in high school not alienating fellow students, then, perhaps, she'd have friends to hang with at the prom (how can you not know that cheating off someone's homework is totally not cool???). I'm not saying I don't have regrets regarding high school (would've spent much more time focused on my education, for one), but I definitely don't regret my level of niceness/friendliness.