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She's wonderful to the point that you struggle to find words to praise her.
Literal
She [topic-は] praise words [for-に] struggle [to-the-extent-程] is.
~程だ ('to the extent that') frames a description by giving its limit — literally 'it is to the degree that.' Here the degree is a paradoxical struggle: she's so admirable that praise itself becomes hard. ほめる言葉 ('words of praise') is a relative clause modifying 言葉 — the verb 直接 modifies the noun without a relative pronoun. The に on 言葉に困る marks 'words' as the domain of the trouble: 'to be at a loss for X.' This is a stock praise pattern, often used about excellence in performance, character, or service.