I think it could be both "boy" and "guy", but uUnfortunately the context is not enough to choice with accuracy:
I'll quote: (http://dcvb.iecat.net/)
1. Very young man, specially the one which has not still reached the puberty. Spanish: muchacho. [...] a) In oral conversations, "noi" can be used to refer somebody of your age or somebody you've known for a long time.