Both kinda have a point, but also both are kinda in the wrong here. Yes the whole thing can have an influence on his future, so it shouldnt be seen as "just some hobby". But at the same time cutting out everything but baseball is also dumb. How many peoplewent full on work-animal in their youth to have "better chances in the future", but come to regret missing out on many things in their youth once that future came to be?
Same for their reactions. Girl, no, he shouldnt just quit everything he likes to spend more time with you. He is his own human being, he has every right to do things he enjoys without you. And bro, i get that you care about your friend, but being about to go physical on your friends GF over a (although a bit dramatic) difference in opinion? No, thats NOT the way to go. The same thing that counts for her counts for you: Your friend is his own man with his own decisions for which he has to bear the consequences. Trying to help / give advice is fine and all, but here you definitly overstep the border.
They should both stop thinking like this is an "either-or" situation. There is no reason to stop seeing her just to focus on rehab, he can get treated while seeing her from time to time without hanging on eachother 24/7. Just get together and talk to eachother directly, all 3 of you. Granted, it would most likely get them nowhere because they will still act the same way with Hiroshi trying to play it of like no biggy, Bro-Dude overreacting and the girl playing the "i hope my meager existence doesnt cause trouble" victim role or going psycho again. But then again, thats how they already act, so it wouldnt make things worse at least.