It's a Peter Singer rationalization and is essentially another version of the shallow pond parable. Instead of destroying a nice pair of shoes, Jeff is, arguably, making the world worse by working on ads. But he's making it more better by using his salary to help people.
I would argue it's the reverse that's true. He's making it significantly worse than his relatively speaking small contributions make better.
If I design advanced weapons, but donate my salary to charity. The good I do is temporary, but the advanced weapon technology can kill, and repress indefinitely.
It seems like it would depend on the details. In your example, I might agree that there is net harm to the world. But in other cases, and maybe Jeff's, the harm done by working on ads may be more than offset by the lives saved.
Since Jeff is a thoughtful guy and this is important to him, I'm inclined to think that he is producing net good. If you can convince him otherwise, he'd probably change what he's doing.
At best his work is annoying to a lot of peoole. At worst, he profits (a great deal, I might add) from a industry that exploits, manipulates, and coerces.
It's not really my or anyone elses job to convince him.
