It's legal to give up your citizenship - he plans to live in Singapore and he must still pay the exit tax. This is what the law says and he doesn't break it.
If you want to discuss this emotionally, try to see his point of view as well: US is the only nation in the world (ok, maybe with 1-2 other exceptions from Africa) where residency doesn't determine the tax authority you're subject to. He plans to live and have his residency in Singapore and pay taxes there but US would still like to have his tax money, who's the ass?
If you want to discuss this emotionally, try to see his point of view as well: US is the only nation in the world (ok, maybe with 1-2 other exceptions from Africa) where residency doesn't determine the tax authority you're subject to. He plans to live and have his residency in Singapore and pay taxes there but US would still like to have his tax money, who's the ass?