They will get the best return dealing directly with a refiner.
Hoover and Strong, David H Fell, Pease & Curran, United Precious
Metals, Hauser & Miller etc. Contact several of them and get the
return rate on silver at that quantity It will be good if they can
get it weighed so that they can get a accurate quote. If it truly is
solid sterling silver and the size of a shoe box it will be quite
heavy so they will probably need a trusted person to help them move
it for weighing.
James Binnion Metal Arts