i think that when lilys mom got sick she didnt really tell anyone and she kept a great facade that only dropped when she thought no one was looking
(spoiler alert: james was always looking) and one evening in sixth year he confronts her about it
at first she is strong and all but she just kind of breaks down
and its okay because he know what she is going through and come on. its james.
and its james who talks her down and lets her know that she is not alone and that she can always talk to him
it think that thats the moment when she truly came to the terms that he changed and thats when they really became friends
Star, harry Louis, sad headcanon