In the waikthrough for g/s/c in the part about blackthorn city it says a girl will trade you a female dragonair for a rhydon but in my pokemon crystal it says she will trade it for a dodrio

Not an error. In Gold and Silver, which the walkthrough was written for, it was a Rhydon you could get in that trade, whereas in Crystal it was changed to Dodrio. The Walkthrough is specifically stated as being for G/S, and in the Crystal Introduction, one of the changes mentioned is that the in-game trades have been changed.

oh ok then