Let them cough
When someone chokes on liquids it usually means they have gotten the liquid into their windpipe and the irritation causes them to cough
It is a reflex to try and get the material out of the trachea and into the esophagus where it should be. Patting on the back or repositioning the head are of no help. Simply let them cough and keep them upright as this will permit the most effective cough.