A friend of mine is currently working in th U.S on a H-3B visa. She has been advised that she can only claim back $602 of the $2335,62 in Federal Tax that she has paid in over the last year... is this possible ?

I believe the visa is H-1B.

If only $602 dollars was withheld from her salary, and she is under the age of 25 at the end of 2006, then, yes, that amount is very possible.