I don't have any problem with what people do in their own homes, nor mostly what they do in public. I have had some experience with adult child roleplaying and have seen situations where an adult child interacts with third parties, which is where I feel like a nonconsensual boundary may be crossed. We have different obligations in how we treat adults versus how we treat children, and in presenting as a minor an adult child creates some ambiguity in what constitutes appropriate behaviour.
I also have some concerns regarding interactions between actual children and adult children, especially unsupervised, and expect that a lot of people might react intensely to their children being with an adult child. I'll have to think about whether my feelings are based on reasonable concerns or irrational fears.
There are some tricky aspects in expecting the world to treat an adult as a minor child. The person in question is not giving up their rights as an adult, so if we were to come across this child unsupervised, wouldn't we be obligated to turn them over to CPS or the police? If the answer is no, then I don't think we truly have an obligation to acknowledge them as children. While we may wish to do so, as a kindness, I don't think it's demanded of us.
That being said, I wish her well and celebrate hew new found happiness.