。
She had no choice but to marry the old man.
Literal
She [topic-は] that old-man [with-と] marry-couldn't-help-but [past].
~ざるを得ない ('have no choice but to') is a slightly bookish, formal way to express forced action — the only path open. It is built from classical Japanese: ざる is the old negative attributive ('not doing X'), 得ない means 'cannot get/obtain.' The literal sense is 'cannot escape not doing X,' a double negative meaning 'have to do X.' 結婚する pairs the marriage partner with と (comitative), parallel to 友達と話す or 兄と住む. The sentence carries an undertone of social pressure or arranged marriage — a familiar shape in older Japanese fiction.