Konote wo Hanasanai

From Sugar Oasis:

Kou and Yukako have been friends since elementary school but over the years Kou has fallen in love with his childhood friend and when he found out that she was moving as a result of her family mounting debts he was devastated at the prospect of losing his best friend and the woman he loved.
That was back in junior high. A few years after their separation, Kou and Yukako meet again. Kou is happy to see her after so many years apart but at the same time shocked at how frail and thin she has become. The strong genki little girl he knew was gone and in her place stood a half-starved homeless girl forced to steal to eat.