Crying af

There's the affectionless father and the overly affectionate mother, and then there's the beautiful 18 year old high schooler Rina.
"Friends are just meant to be used," is what Rina says when she is caught having sex with one of her friends' boyfriends. Rina has such a low opinion of friends, but when she is hospitalized her classmate from elementary school, Maki, appears before her, wanting to continue their free-of-charge friendship from long ago. In this day and age, just what is the meaning

Moriyama Rin was born with an incurable disease. Despite her best efforts to live a normal high school life, Shimehara Satoshi was the one who ruined it at the very end.
'I wish I didn't say I liked her.'
Satoshi, full of regrets, experiences a time leap and meets Rin for the second time.
'If I don't involve myself with her, Rin will be able to spend her short time left and end it with a smile...'