Answer:
Friendship is causing a big change in both Mary and Colin.
One way friendship has changed Mary is that she is now kind ad empathetic towards individuals.
I know this because on page 134, Burnett writes, “Somehow she was sorry for him and did not want him to lie awake, so she leaned against the bed and began to stroke and pat his head and sing a very low little chanting song in Hindustani.” This shows that that she can learn to not be so self-centered and mean, like she was when she first moved there.
Likewise, friendship has made Colin more open-minded than he was before.
On page 205, it says "A little later the nurse made Colin ready. She noticed that instead of lying like a log while his clothes were put n he sat up and made some efforts to help himself, and he talked and laughed with Mary all the time." In other words, Colin had gone from being a helpless crude young prince who never did anything for himself to a young boy who doesn't mind trying to help himself if.
By examining the effects of friendship on Colin and Mary, it is clear this author thinks friendship is very important to have when your young, and lonely.