It's common to want something that's out of our control.

In this moment, a friend is moving away. Your kid wants them to stay, but that doesn't change the fact.

Epictetus said the extra suffering is the fight, the thinking that wishing hard enough will make the friend stay.

What is up to them is smaller: they can keep a photo, draw the friend, sit with you.

You do this too. You cannot keep a friend from taking the job in another city, but you can call them on a regular cadence.

Do not tell them not to miss the friend. That is not the lesson. The lesson is that wishing does not change the fact, and they are still allowed to be sad.