。
She warned him not to go alone.
Literal
She [topic-は] he [to-に] [alone-1人で] go-not [such that-よう] warned.
~ぬ is the literary/archaic equivalent of ~ない (negation), still alive in fixed expressions and formal writing — modern speech would say 行かないように. The pattern ~ように (clipped to ~よう) introduces the content of the warning. 1人で uses で in its instrumental/manner sense ('by oneself').