Its arguable if potassium supplementation does anything at all. Search for it, there's LOTS of threads about it. I'll summarize.
Natural seawater is about 380ppm. Most artificial salt mixes are much lower than this. Potassium is important to all cellular life, however, there is no sink for it as there is with calcium or magnesium. Potassium is present in the in foods we add to our tank also. Many very knowledgeable reefers are skeptical of the need for, or benefit of, potassium supplementation.
I chased the white potassium rabbit for a while myself. I tested my tank's potassium and it was at 235ppm. I supplemented for weeks with K-balance and saw no change. Recently, I bought some beautifully colored frags from another reefer, tested his water, and it was even lower in potassium than mine! Plenty of other reefers have great color without worrying about potassium. Personally, Im pretty well convinced its not necessary and there are much more important things to attend to that will get you better color.
If you do choose to supplement potassium, there are a few ways to do it. From what I have read, potassium chloride is the easiest. Potassium chloride is available from health food stores in powder form relatively cheap, or as pellets for water softeners (but purity is still in question with the latter). There is also potassium iodide and potassium sulfate, and many others.
As for the supplement your friend is using, I see this: "Ingredients: Filtered Water, Potassium Aspartate, Potassium Citrate, Sodium Benzoate". Its mostly water. I would choose other means of supplementing potassium before I chose this... Im not sure what the aspartate, citrate, and sodium benzoate components of this would do.
If I were to supplement potassium, I think I would simply use potassium chloride to correct my fresh mixed saltwater to natural seawater levels and leave it at that.
