that's going to be a tough one based on location and since '03s can not be CPO'd by dealers any longer. As such, it's not real desirable from their business perspective and essentially is just any old used car. Unless it's immaculately clean and w/ good records, it will be wholesaled out to a private jobber or auction. That being said, tell your friend to be well prepared to feel insulted (unless he wants to pay full boat on what he's buying).
If trading at a branded dealer, I think your friend can assume he'll get offered ~ 60-65% of what 'they' are asking for on used 2003 w/ comp miles. Based on a quick PCNA search, there aren't any '03 S models for sale. '03 base models are basically showing an asking price of $20k. My quick # is add $1500 for S, less $500 for miles and multiply it by 60-65% to get roughly $13k.
Curious to hear what he actually gets offered...