Let's say someone starts to mock him rudely. He's defenseless, and you know he won't be able to stand up for himself.
There are several possible courses of action.
You can join in and start making fun of this person, laughing your head off.
On the other hand, this is a good opportunity to show yourself, to stand up for him, and gain popularity.
And, as always, you can ignore it and not pay attention, taking a neutral position.
In my opinion, the second option is the manifestation of morality. But is it really? If you stand up for him, there will be a selfish motive behind it – the desire to show yourself off and elevate yourself in the eyes of some members of the group.