The "handyman" burned the concrete by using too strong a mix of acid. Prosoco sells two products (the names escape me) that will reverse the burn. Before bidding on their job, you will need to perform a test. Most brick yards will sell what you need. Follow the directions exactly and you should have good results.
Since you don't know which product will work, you may want to buy one of each - and they are around $25/gal. So, unless you are willing to pay for your own education, you will want to charge the customer for the test. He may decide that the drive doesn't look so bad after all.
P.S. In my experience, the stains will not fade significantly over time.