My friend is an artist and she wondered if she can draw on the walls in Amsterdam legally and if yes where?
No it is not legal (not just Amsterdam, but everywhere). Some municipalities have 'walls of fame' where it is legal. (They are sometimes called 'Hall of fame' (HOF)).
Here is a map of them, but I have no idea how up-to-date this is. It mentions one under the Schellingwouderbrug (near Flevopark) in Amsterdam, but it's current status is unknown.
Website legal-walls.net also mentions some Amsterdam locations (again mentioning the Flevopark - read the comments), but again, no guarantees here. In The Netherlands we sometimes have the tendency to tolerate things that are officially forbidden.
Reddit thread on the subject.
Note that the fine for graffiti in The Netherlands is € 140.=.