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It's obvious to any onlooker that she's head over heels for him.
Literal
She [subject-が] him [to-に] in-love [topic-は] bystander's-vantage doubt-of-no place [copula-だよ].
惚れてる is the casual contraction of 惚れている ('is in love with'). 岡目八目 (おかめはちもく) is a 四字熟語 from the board game Go — a bystander watching a game can see eight moves ahead that the players miss. Here it means 'obvious to an outside observer.' 疑いのないところ ('a place of no doubt') reinforces certainty. The sentence uses a sophisticated Go metaphor for everyday romantic observation.