P.W.
There's a kind of "paint" that kids use to paint on cars. It's meant for that purpose and actually comes off easily. Hopefully they used that kind of paint on your friend's car.
I know about that paint because my daughter and her friends just went out "painting" cars a few nights ago. It's warm here, so the paint freezing on the cars is not an issue. They only do it to their friends, so it's not vandalism. The paint washes right off easily.