Depends upon the training level of the lifter. A novice is going to make gains on just about any training schedule. 2 days may have some good results if your intensity and progression are appropriate. 6 days a week is possible as well, but I pity your brother, because I tried it for about a month in college and couldn't keep up the intensity. keep us posted on the progress both of you respectively make.
"I have one book for you, Starting Strength; Read IT"
You can't kill a man who refuses to die.