、。
If he happens to be busy, help him out.
Literal
By-chance he [subject-が] busy [explanatory-のなら], help [command-なさい].
のなら (or んなら) is a conditional with explanatory nuance — 'if (it's the case that) X.' Used when responding to information or accepting a premise about the situation. なさい is a mild command form (politer than plain imperative, firmer than てください), often used by teachers or parents.