A friend told me they did this recently, and I argued to her that it was a bad idea. It's going to sit around for however long with all the baddies in it multiplying, and then you're going to smear it all over your hands to "clean" them. This was my feeling mostly because she had made it with just tap water and dish soap, but I imagine even boiling the water first wouldn't stop this happening as it's going to sit around as still water for weeks/months or even longer.

Maybe I was over reacting a bit, I'm admittedly not sure how store-bought foam soap is produced. I just search around quickly and see that the prime ingredient in most foam soaps is (unsurprisingly) water, but they're also pumped full of other stuff.