、。
She's very upset because her cat died, so leave her be.
Literal
She [topic-は] cat [subject-が] died-and very is-grieving [because-ので], [gently-そっと] [leave-as-is-しておいて] [for-her-やりなさい].
Several layers. 猫が死んで is a te-form-cause clause ('because the cat died'); 悲しんでいる is ~がる-equivalent for grief, in ongoing form. The whole reason is wrapped with ~ので. The directive そっとしておいてやりなさい stacks: そっと ('gently') + しておく ('leave in a state') + ~てやる ('do for someone, downward') + ~なさい (mild imperative). Each piece adds a layer of consideration.