Ugh.
Why did he say you needed a new chain? That's BS, unless you had an 8 speed chain on 9 speed cogs or something.
I'd take the bike back, and let them know what happened, but I wouldn't expect to get anywhere with free wheel repair. If the mechanic is the type of guy who will replace a brand new chain with a brand new chain, he's probably also the type of guy who will weasel out of repairing a wheel for free.