I would definitely agree. The 'gmail' part didn't strike me as out of place so much as the 'zackster' part. I believe Zackattack's name is 'Zachary Burt', which would make 'zackster' an informal (and, in my opinion, somewhat frat-ty) nickname.

The only situation I could imagine that would make 'zackster' appropriate for someone looking for a job would be if his first name were Zack (or Zachary, or some other derivative) and his last name were 'Ster.' Since I doubt that is the case, I suggested he set up some other address for soliciting business

Since Zack is just graduating it will be (relatively) easy for him to make the change now. It will likely be more difficult in a few years once his name/email get out there.

Agreed. I have a personal account and a professional Gmail account. My professional Gmail account isn't mapped to my URL; it's just my full name @ gmail.com; the personal is this username at Gmail. Not quite as informal, but still not business appropriate.