I have no experience with a '90 Corvette but I think your friend has a couple of options. Mufflers from different manufacturers have different sounds, and some may not be a tone he likes. He should go to the websites of muffler companies like Magnaflow, Flowmaster, Corsa, etc., and see if they have an audio function so he can hear the tone each brand makes.
Many of those companies make complete exhaust systems and companies like Corvette Central, Ecklers and others sell complete systems as well. Another option is to go to a local speed shop that fabricates exhaust systems...specify what muffler brand and get a quote to compare prices.
Note that licensed muffler shops probably won't do anything that affects emissions negatively...unless the car is used for "off-road" purposes.