You’d have to find out about the rules that her school give for prom etc
At mine we were not allowed to invite people to come with us that were not a member of our year group
However if it is allowed, it’s not that big of a problem, you’ll have fun together and ignore what others think. You won’t be the only older person there if they are allowed to invite people I guess. But at the same time, she needs to be able to have fun with her school friends as well as spending time with you, it’s the end of a long time spent with these people that she might not see again, and she needs to enjoy the experience of prom with her friends as well as having you there if that makes sense