。
She denied that John had helped the boy who'd lost his way.
Literal
She [topic-は] John [subject-が] road [in-に] got-lost that boy [object-を] helped [thing-of-こと] [object-を] denied.
A nicely layered sentence. The relative clause 道に迷ったその少年 ('the boy who had lost his way') modifies 少年, and the whole resulting clause 'John helped the boy who had lost his way' is then nominalized with こと and made the object of 否定する ('deny'). Note that within the embedded clause, the subject is ジョンが — using が, as is standard for subjects of subordinate clauses, since は is reserved for the main topic. 道に迷う is a set phrase where the place 'where one gets lost' takes に, slightly counterintuitive for English speakers who'd expect something equivalent to 'in.' Two layers of embedding plus a relative clause makes this a solid grammar workout.