You can also refer to it as describing someone else renoobed, or renoobified.
According to the definition of noob, the word can also be used as 'renewb' , 'renewbie' and 'renewbified'.
"He used to be so damn good at Halo, but now he's a renoob!"
"Man, he used to be so good at pool - but now he has renoobified."
"Man, he used to be so good at pool - but now he renoobed."