How can it be done?
بصعوبي
Which language?
LOLCODE
On a serious note, I am assuming since that guy wants a touch based Captcha that means he wants for either mobiles or IPad so it can be basically done with most of the languages used for mobile development such as Objective C(for Iphone), Qt C++(for Nokia), Java(for Android) etc...
What is basically needed is some kind canvas or graphics box that acts as a drawing area and can take input and draw. That area would contain a certain shape drawn previously by the program in some lighter color like light gray and then the user will have to draw over it (with another color). One the user finishes drawing over that shape he/she would click a button for submit the form and so it is submited and validated by the server.
That's how I basically see it working (in terms of scenario). As for the validation of the swiped shape I am not sure but most probably by keeping track of some coordinates in the generated shape. Once the user is able to go over these points in his swiping it would be valid.
I am not sure how much this can be secure but I doubt the security and efficiency in this idea.