The only way to counter this is to claim the same thing of Webb. With his background he has certainly used it more often than George Allen, and with his on the record remarks concerning the Tail-Hook scandal I guarantee there's a woman some where that he called an ignorant "c" word.
This is a charge that can kill damn near anyone off. There are about six English speaking white men on the planet that can claim they never used the "n" word, so when Allen claims otherwise people just roll their eyes. It's like asking someone if he's quit beating his wife.