1.  Ask your client about the assets and liabilities he has on 1/4/2012 (Assuming that the accounts are for 2012-13 FY)- this will be your opening balance sheet.
2. Based on that prepare detailed accounts in tally till 31/3/2014. Cash balance will come automatically.
It is necessory to invest some quality time in making the opening BS as no asset or liability should be missed out.
(If you prefer to file return u/s 44AD, show that cash balance as any reasonable amount which the client keeps in his office+ home at any day. Also check his bank a/c, if he has made large deposits in the 1st few days of the next month, that will be your cash in hand)