。
Who is she talking to?
Literal
She [topic-は] who [to-に] is-talking-to [explanatory-のですか]?
話しかける ('to talk to / address / strike up a conversation with') is a compound verb 話す + かける ('begin / direct toward'); this kake compound is productive — 笑いかける ('start laughing'), 食べかける ('begin eating'). The recipient is marked with に. The polite question ending のですか hedges the inquiry: it implies 'I notice she's talking to someone — who is it?' rather than a neutral information request, which would be 誰に話しかけていますか.