。
She's loose-lipped.
Literal
She [topic-は] mouth [subject-が] light is.
口が軽い ('light-mouthed') is one of Japanese's body-part personality idioms: a light mouth lets words slip out too easily — 'can't keep secrets, gossipy, loose-lipped.' The opposite 口が堅い ('hard-mouthed, tight-lipped') describes someone trustworthy with secrets. This idiom family runs deep: 口が悪い ('foul-mouthed'), 耳が痛い ('cuts close to home'), 顔が広い ('well-connected'), 足が早い ('quick on one's feet'). Each treats a body part as the seat of a personality trait, an extremely productive metaphorical system in Japanese.