Here's a few options.
"All elves can become human, and this one chose to". I mean, if it was
good enough for Tolkien, who am I to argue? For this to work, the character was elven the whole time, but is choosing to become human in order to... well, I guess that depends a little more on plot. Get back at mom and dad, no, this isn't a phase. See what lies after death, because she's been convinced by the party cleric that it's an all-the-times party. Dodge an arranged marriage.
"She was in disguise the whole time". Others have suggested it, and it's certainly the lowest impact solution. Wasn't an elf, just liked elven fashion and had a slight build.
"Elves are magical; an elf who lives in cities or amongst humans is a human". Elves that spend too long in cities, or which swear fealty to a human noble, lose claim to their fey heritage and become mundane humans. Well known phenomenon (sort of similar to the first option, but caused not by the positive choice of mortality but the passive acclimation to the human condition). If she spends some time among elves again, she (like any human with a glimmer of fey ancestry) would become an elf again.
In my campaign, dragons are teratogenic; creatures consuming dragon blood become dragonborn (or birth dragons, or become dragons). If the player is interested in that, maybe something similar could happen here. But I don't see exactly how that would work for turning into a human; much easier for turning into a dragonborn or a tiefling or something.
"Don't try to make it work, it's just not that important". Comic books do it all the time. Just retcon it.