The current balance isn't necessarily supposed to match the current balance in the cash account. If there are uncleared items, it should match the cash balance as far out as you can go date-wise for the cash account. If they are on a calendar year, look at the balance at 12/31/2017. Assuming they don't have anything posted to future years, that number should match the current balance in BR (unless he messed with it). If it's still off, backup your BR folder, rebuild the BR files, then run the ""Recalculate Account Balances"" utility in BR. If the reconciliation is off after that, it's probably something that was posted to the cash account through GL, or something posted to BR that did not post to GL.
If he's so fond of QB, just have him adjust any differences to a ""MIsc. Customer"" account. 9 out of 10 times, when I convert a new client from QB to Sage, there's always some huge, unexplained AR or AP amount, that gets carried over year to year. No one ever knows where it came from, or why its there. :)