。
She's practically a thief.
Literal
She [topic-は] thief same-as is.
~同然 means 'equivalent to / practically the same as' — 泥棒同然 = 'practically a thief / as good as a thief.' This is a blunt, judgmental statement. 同然 is a formal suffix that intensifies the comparison — it's not 'like a thief' (ような) but 'as good as a thief,' implying her behavior is indistinguishable from theft.