I mean I get it if it's like a normal friendship, I also haven't seen people in ages but they weren't important. In this case for Rinko, Haruno was her guardian for some time and the guy who took care of her when nobody did; for Haruno, Rinko was the girl who gave him hope in life and a push to change, but in this case Haruno decided to cut contact for absolutely no reason, he could start a new life and check on them from time to time or contact the mom, bet they would have been happy to know from him. Like, it wasn't a normal friendship, it was the love you have for a family member, they were important for each other.
That's why I find weird the author's decision of completely cutting off the contact for more than a decade in such deep relationship that easily, in a single day.
Idk, maybe it's a me thing or cultural difference since in my context you don't let go such important relationship that easy in life, I even stop seeing old friends from high school for a whole year but a message or some drinks from time to time it's normal.