If it has an established growth pattern, you may be out of luck. Why does the client want it "straightened"? (This could be a danger sign, by the way. My one client obsessed with "straight" trees has major control issues.)

If it is young enough, it may be possible to re-dig the rootball and replant the tree so that it's straight. Then, with careful guying or staking, it can be trained to grow in a more "pleasing" direction. This should be done in the fall after the leaves have dropped.

You may also need to do some pruning to get it to grow in a more uniform way. Is it leaning because it's searching for light? If so, you may need to prune its neighbor. Or, it may just be the wrong plant in the wrong place.