mjbn, I think it is unfair to tell him that he needs to do daily water changes without knowing what size "bowl" he's using.
I've only ever kept my Bettas in bowls and they only ever died of old age. :) The key is making certain it is a large enough bowl (1.5 gallons or larger) and keeping live plants in them. The live plants create a natural cycling process that replaces a filter in a smaller environment.
Also, you need to be very aware of the air temperature in the house as you can't use a heater in a smaller environment. I can often curb this problem with the fact that I am naturally cold and my house is usually in the mid 70s anyway, plus I use a flourescent plant light for my bowls since they house live plants. Even though they are flourescent, plant lights generate a generous ammount of heat.
I add water to my bowls throughout the week as needed and do a weekly change of 100%.
It might also be relevent that I am very careful about feeding. My fish get fed one pellet at a time. Not only is this good interaction between me and my little friends, but I'm absolutely certain that there is no excess left in the bowl to rot and pollute the water.
Again, I've never had a sick Betta.
Note: I only say this about 1.5 gallon or larger. If you are keeping it in anything smaller. . . GET A BIGGER BOWL! :P