Cuboholic
A person addicted to playing with his/her Rubrik's Cube. This addiction was at its peak in the early 80's, shortly after the introduction of the Rubik's Cube. Also, Cubaholics Anonymous was founded in 1980 by self-confessed Cuboholic Augustus Judd.
If your cousin is playing the Rubrik Cube for more than two hours a day, it's safe to call him/her a Cuboholic..