The simple answer is no, the e-file system is not a bank that accepts installments. You can schedule estimated tax payments for the future year, but the prior year tax allows for 1 payment. 

You have advised your client that the tax is due, and delaying results in P&I. You could make the payment thing work by filing for an extension, paying the 50% with the extension payment. Then come back later and file the return with the second 50% payment. While not recommended, this is a potential workaround.