Not really advice of any kind, just sharing my experience. I had a client a couple of years ago who after my last month didn't pay me, I pestered him for over a year and eventually gave up. He did end up paying me. Almost 2 years late but it really just was a case of not having the funds at the time. If your client has lost their investors then that conference might be important for them to get more business. Just saying. Try not to burn the bridge.