Email

So my brother wants to cancel his email address he used when originally purchased GW. Will that effect his ability to play?

No. The e-mail I created my acct on hasn't existed for over 3 years.

If anything that makes it less likely that your username will get phished.

I would advise you change the E-Mail address for the account before he does it.

The e-mail address he originally used is now also the "account name". It doesn't matter whether or not the account name is valid e-mail address (once you've done the original account creation).
So, if he cancels that e-mail address, it won't affect the game, and he will still use the same "account name" (which looks like his old e-mail address) when logging in.
Be sure to add the new e-mail address to your GW/PlayNC account. Otherwise, any confirmation e-mails, such as to change your password, will still be sent to the old address.