These sorts of old wives tales, or since this is your dad, folk logic often have a grain of truth in them. But its very difficult to find what that truth is -- given that you "just shouldn't" with no explanation of what will happen.
Consider: you've mixed a cup of tea and a cup of water, into your stomach, which has a fixed capacity that you can find on Google. Eventually the contents are diluted across your entire mass of 90 kg. What happens when you dump a cup of tea and a cup of water in a bowl? When you dump it into a bathtub? You can try to replicate the existing environment: have 0.1 M HCl there to mimic stomach contents. This is commonly done to determine dissolution of pharmaceuticals -- we can replicate stomach contents with HCl, pepsin, deoxygenated water at 38 ° C, mucus -- even mammal digestive juices if the experiment requires it. What reaction can we possible expect from mixing tea, dissolved in water, with more water? What will we test for?