There are wood bleaches on the market, but they should only be used on hardwoods. Wood Bleaches are quite caustic and can damage some woods. It will distroy pine and fur, and poplar doesn't have a chance in the world against it. I've had success in using normal household chlorine bleach, but that is a long drawn out process on something as large as a staircase. The trick with chlorine bleach is to keep it wet with undiluted bleach for several hours. Either process will raise the grain of the wood an will then require a lot of sanding. Like Steve said, "Detail sanding", hours of it. Any process like this should be tested in a small area first, before you attempt to do the whole thing.
I'm with Steve, if this person is any kind of builder at all, he/she should make it right. I was taught that it's ok to make a mistake, it's no ok to leave it that way.