Shigatsu wa Kimi no Uso

[Kraco]

I don't really understand the MC's friend. What exactly is his relationship to this girl?

He's a huge womanizer, plain and simple. He'll take all cute girls interested in him (and there seem to be plenty). Actually I think that's jolly good because under such circumstances it shouldn't be so bad for him when the MC "steals" the violin girl away from him.

Childhood friend is playing a dangerous game. She likes the MC, but wants him to overcome his trauma and play piano again, so she is introducing him to this girl that he is sure to become infatuated with.

She's playing such a losing game. Though I'm not altogether sure how well she has played the game at all. It's like she only loves the piano playing MC, the one not playing is nothing but a neighbourhood friend. Basically, she has wasted a lot of time and made the MC get used to her as a childhood friend, not a member of the opposite sex.