Cleve Moler, one of the founders of MathWorks, has written two free books. The first, Experiments with MATLAB, might be a good starting point: http://www.mathworks.com/moler/exm/index.html. It teaches some maths as well as Matlab.
MathWorks also has a relatively new set of online problems that she can work through: http://www.mathworks.co.uk/matlabcentral/cody/problems. In particular, the Cody Challenge problems start out very easy, but cover a range of language features. After doing some problems, she can see other people's solutions, and hence pick up on more idiomatic ways to do things in Matlab.