My original sentence was:
[Name]さんは仕事でよく忙しいんですか？
I was told by a native that it sounds strange. She replaced よく with いつも:
[Name]さんは仕事でいつも忙しいんですか？
I always thought both were used for "often" frequency.
Why does my sentence sound strange? Her explanation was long and in Japanese above my level so I didn't quite get it.
Your sentence:
「[Name]さんは[仕事]{しごと}でよく[忙]{いそが}しいんですか？」
is strange because 「よく」 is most naturally used to modify verbs, and not adjectives like 「忙しい」.
Thus, 「よく忙しい」 sounds just very strange to the Japanese ears as our ears are not used to hearing such word combinations. In other words, if you literally translated the English sentence "~~ is often busy." into Japanese, it would not work.
「いつも忙しい」, however, sounds just fine because unlike 「よく」, 「いつも」 modifies both adjectives and verbs with equal ease.
So, if you want to use 「よく」 to say how busy a person is, you must use a verb somewhere as in:
「～～さんは仕事でよく[出張]{しゅっちょう}します。」 = "~~ often takes business trips."
「～～さんは仕事が忙しく、よく[深夜]{しんや}に[帰宅]{きたく}します。」 = "~~ often returns home late at night as he is busy with his work."
よく is a derogatory term, often uses to what you don't want to happen, like:
台風がよくきますね
