It is not easy to convince someone who wants to believe otherwise. In my experience, no amount of detailed scientific information will help in this regard. People attracted to charlatans often interpret such arguments as "conspiracy of the scientific establishment".
However, seeing (or tasting) is believing, and what usually works well is experimental evidence. Make a double-blind test, where you let try your friends ordinary water and "structured water", and let them evaluate the difference (if any). Double-blind basically means, that neither the tester nor you (nor anyone else in the room) should know which is the "structured water" before the evaluation, to avoid some psychological bias. If you do sufficiently many of these tests (say, 20 probes for each type of water for 5 different persons, so 100 probes in total) you should get nearly a 50% chance that the tap water is identified incorrectly as "structured water" or vice versa. So if in the end only 40-60 of your probes are identified correctly your friends know that the structured water claim is bogus, since flipping coins to decide which water is which would have achieved the same result.
Beware, however, of slight temperature differences, particularly if the test consists in drinking the water. All physical and chemical conditions should be identical if you really want to test the (non-)effect of giving the water "structure".
At some fair the clever guys selling the structured water cooled it slightly below the tap water (about 1 Celsius less). Thus, the "structured water" tasted more fresh than the tap water.
As a historical aside, in Austria we have a similar issue with so-called Grander-water - funnily, Mr. Grander even received the Austrian cross of honor for scientific merits from former minister Gehrer. So even science ministers are not immune to such charlatans.