這句文法是否有錯？

看來，你對時式的了解很透徹，不錯，如你所說，過去完成式past perfect tense，確是先發生的動作用had+past participle，而後發生的事，用簡單過去式。
也正如你所說，They had been given help and advice before they made the decision.才是正的說法。所以你的觀點是絕對正確

順便一提，在英文裏，卻兩個動作，若以連接詞聯上，其連接詞又能清楚表達時間的先後，如：after, before, then等等，就兩個動作均可用簡單過去式，如：

I arrived at the station after the train started. (= I arrived at the station after the train had started.)

所以，為求簡潔，你的句子也可以改為

They were given help and advice before they made the decision.

希望幫到你！

我覺得又無錯喎 They had been given help and advice before they made the decision 依句。

They were given help and advice before making the decision.

They had been given help and advice before they made the decision.是不正確，句子應改寫成They were given help and advice before they made the decision.