What does the idiom his friends explained that they had been shaken up means?

The idiom "shaken up" in this context means disturbed, upset, or emotionally unsettled. It suggests that his friends were affected by something emotionally challenging or traumatic.

Here's how to break it down:

* "Shaken up" implies a sudden and forceful disruption of their emotional state.

* "They had been shaken up" indicates that this emotional distress was a lasting effect of something that happened previously.

The idiom implies that the friends experienced something that left them feeling unsettled, whether it was a personal loss, a frightening experience, or a difficult situation.