。
She was completely taken in by his gentlemanly facade.
Literal
She [topic-は] his [genitive-の] gentleman-like [adj-らしい] outward-appearance [by-に] ended-up-being-deceived.
Two grammatical patterns reinforce a tone of regret. ~らしい ('-like, characteristic of') is the conjectural/seeming use — 紳士らしい体裁 ('a gentlemanly appearance,' implying 'looks like a gentleman but isn't'). ~てしまう is the perfective/regretful auxiliary, marking that something ended up happening (often unfortunately, embarrassingly, or finally) — だまされてしまった ('ended up being deceived,' with regret). Together: she was taken in completely, and the framing makes that fact rueful. 体裁 ('outward appearance, façade') has a slightly negative connotation — the façade vs. the truth.