1) you had a recent backup of your wallet.dat
2) you erased everything, reinstalled bitcoin core and replaced the new wallet.dat with your backup
3) you see all transactions, but have 0 balance
First question: is the wallet sync'ed? If not, you'll probably have to wait untill the wallet is completely synchronised (this might take a couple of days).
If he sees his transaction history, it should be synced up to that point and not show zero balance if there were funds before.
First, you can check your public address on a block explorer to see if the funds are still actually there and have not been transfered by a hacker or something.
Next, you can do a -rescan to update the correct balances.