。
She can't handle the heat.
Literal
She [topic-は] heat [to-には] weak [explanatory-んです].
~に弱い (weak against / can't handle) is a useful pattern for expressing vulnerability to something — the opposite is ~に強い (strong against / good with). The には here combines the target particle に with the contrastive は, subtly scoping 'heat' as the specific thing she's weak against (implying she may handle other things fine). The んです ending provides explanatory context, as though clarifying a situation: 'the thing is, she's weak with heat.'