Complaining about a large download for a retail disc is perfectly legitimate. The whole point of getting a disc nowadays is so you don't have to spend hours or days downloading on a slow connection - otherwise there would be little benefit (unless you have a paper manual fetish).
The key issue is whether the retailer and/or packaging made the reliance on a Steam account (and all that goes with it) clear. If they did, then the fault lies with the purchaser - if not then returning it for a refund would seem the most appropriate response.
Steam requirement (both online connection and free client requirement) is written and visible on the back of the DVD case.
