Client not paying remaining 50%

Was there a contract involved? If so, these kinds of things should always be included in the contract, and if not, well I guess it's easy to see when contracts can come in handy. :P

I think giving a notice of some sort is a very smart way to handle it, just give the client enough time to see your message and handle it, in case they are just running short on money. In the future, be sure to run out any opportunity of a client doing this to you by specifying payment due dates for one, and even fees/penalties for late payments.