のに忙しかった

was busy doing

→ の
phrase Rare #24,517
was busy doing Used after a verb in the dictionary form to say that someone was busy doing something. Here, のに means “for doing,” not the contrasting conjunction のに meaning “although.”
JA: 彼はレポートを書くのに忙しかった。
EN: He was busy writing a report.

生徒達は学園祭の準備をするのに忙しかった

The students were busy preparing for the school festival.

メールしたのに忙しかった

Even though I emailed them, they were busy.

— 最強のふたり (2011)

母は夕食の準備をするのに忙しかった

My mother was busy preparing dinner.

彼は友達のノートを写すのに忙しかった

He was busy copying his friend's notes.

少女は友達にお茶をいれるのに忙しかった

The girl was busy making tea for her friend.

彼は報告書材料を集めるのに忙しかった

He was busy gathering information for the report.

彼女は宿題をするのに忙しかった

She was busy doing her homework.