jo was so reluctant to write more women in harry potter that she made it canon that the lily's only friend was snape.
we see girlfriends around her but no one ever mentions who they were. mary macdonald? she was probably just a roomate, like lavender was to hermione.
no one in the 90s tells harry how they used to be best friends with lily. everytime remus or sirius brought up friendship, they only spoke of james.
what gives? in fact, in almost the whole series, no one is even checking for lily.
i think this isn't noticed or why the impact of snape's plot twist concerning lily isn't treated as it should because of the films twisting what is and isn't canon. she's mentioned a lot more in the films that the books. the books emphasis is always on james.
lupin saying lily was at one point the only person looking out for him? movie. in the book when harry mentions hearing her literal death scream lupin just says "yOu HEaRd JaMeS?!" he completely ignores lily. francis the fish and slughorn? made up for the movie.
so when snape gives harry his memories, we see her properly. something no one has ever done before in the series. he ironically ends up doing this twice. it's also snape who shows both james and lily for who they really were, real people vs everyone's rose tinted "do not speak ill of the dead" renditions.