Unless you mean to imply that any sexual acts between a young person and a remarkably older one ought to begin with the former showing their ID to prove they are in fact of age, the fact that she was legally ‘a child’ is irrelevant to the moral failing at hand. (Because it would be absurd for the morality of the act to radically shift based on whether it takes place a couple of months later or earlier.)
I do agree it was a significant moral failing (assuming he did go through with it), even if she did indeed present herself as willing and appeared of age. I don’t agree that trying to differentiate it from other related moral failings (even if it’s in a better light) constitutes a moral failing in itself, though. It is beside the point and not helpful to the matter at hand, but I don’t see much harm in it.