Really. Seems obvious
-First friend. Discriminated against by all other peers in youth.
-Taught her how to defend herself with incantation less magic. She is probably dead by now without that knowledge.
-One of the strongest and most highly revered people in their location.
-Befriended her again without knowledge of her identity, nothing but positive interactions between them like every day for over a year.
You are probably looking at this from a modern day lens rather than the setting that these characters are in and the different expectations they have out of relationships.
Who says Ariel and Luke are kind people?
They want to protect her as a friend but it's not like they're so much better.