I don't know a thing about a 2003 Altima (I have a 1996 Altima) but I do know that if the oil is gone, the pressure switch in the system will close and illuminate the light. If there is no oil in the system and the oil light did not come on, the the switch is faulty and I'd start talking about warrenty issues. I'd also do a visual check myself and see if there is oil(wipe the dipstick on a rag to see if it is wet, as very clean or new oil may not show up on a dipstick), buy a bottle of slick 50, do an engine change and Substitute the Slick 50 for a normal quart of oil during the change and have the dealer reset the Check Engine light. Drive the car around and see if the "Engine Blown Code" (Is there an actual code for blown engine?!? or was it for no oil?) If your engine was truly gone, your wife would not have been able to make it to the dealer, or she would have noticed some serious noises and vibrations on the way there. It's possible that the error code/sensor/system is faulty and your engine is actually fine. I'm guessing your dealer said something like: "Look, a girl, lets see if I can get her to buy a new engine. You know how women are..." I hate that. Pretty much why I go to Carmax and not a dealer.
I also like the look on their face when I let them know I'm an aircraft mechanic after I let them give me a c*ck and bull story about how my muffler bearing, fencer valve or flam shaft needs to be replaced. I hate dealers and only deal with them when aftermarket parts aren't available. I realize that the general public can be pretty ignorant about cars, but that's no reason to take advantage of people. Oh, don't get me started.
There are a number of other things to try, but if the engine is running smooth, it isn't 'blown'. Tell the dealer you need more info as to why there is no oil in the system. Is there evidence of a leak? Is the oil pump making rated pressure? Is there continuity between the points of the pressure switch when the engine is off? You can also check this by turning the key to run, but not start. The oil light/symbol should be illuminated. Once the engine is started, the light will go out. Turn the car off and then back to the run position (but don't start the car). The oil light should come back on within 30 seconds.
Good luck and please post your results!