surely if his tank is heavily planted then they will use much of the nitrates up anyway. I don't think hes daft so i don't doubt him using the kits properly. They are not difficult to use.
Have you added anything new recently? in the last few weeks?
I cycled my tank with a Bristlenose Plec and 6 Zebra Danios.
My danios are immense and they are still going strong (and growing and growing and eating soooo much!) - so I think they've got to go on the list somewhere hehe ^^