The color leached from driftwood is called "tannin" and while it is not very eye-pleasing, it does no harm to fish. Infact certain fish, such as tetras seem to thrive in tannin-filled water.
I do feel the need however, to stress that when adding new items to an established tank, you need to make sure they are "clean". There is really no telling what kind of "yuck" might be hiding inside used things. Especially if you have no idea what the health of the tank was when he had it set up. I agree with catfish1102... if the piece is too large to put into a stock pot and boil for a few hours on the stove... take it outside in one of your aquairum buckets, and pour boiling water over it for a few days, several times a day even. I have done the same with used gravel, and other inherited items, or things bought from yard sales.
Better safe than sorry, like somethingfishy said... nothing is worth the health of an established tank.