The US permits dual citizenship, without treaties, by default. If a person is a US citizen, it does not recognize that person's other nationality. This simply means that it's ok if you are American and French, but the US treats you as American, and ignores the French bit..
My understanding when researching my own dual-citizenship is that you do not loose your American nationality when acquiring another nationality unless it was your intent to do so. Some countries require that you do this as a condition of nationalization (and some make exceptions for marriage, previous family ties, etc). So if you acquired a second nationality in a country that required this, knew that it meant giving up your US nationality, and willingly did so, you would no longer be American. Whether or you or the second country mention this to the US authorities is a totally separate matter, which can result in some interesting consequences later.
So if you need to renounce your previous citizenship as a condition of becoming, say, Columbian, then you would no longer be American (probably). Seeing as though he became Columbian through presidential decree, I'm guessing Columbia didn't make him do this. So now he is a dual-national, but from the US government's perspective he is American.
I've read a bit about cases where even if you renounce your US citizenship to the foreign government to become a new nationality, you can still tell the US Govt 'I was just kidding to those foreigners', and you're fine. It's only really an issue if you renounce it to the US Govt directly.
Yes I've also heard about people 'renouncing' their American nationality to foreign governments, not informing the US, keeping their passport, and not having any issues. I've also heard about people exaggerating facts to ensure their children (born abroad) obtain US citizenship (possible if the parents are American and have lived in the US for some amount of years - there's the catch).
The problem is that you really don't want to be in a limbo state with your citizenship. Some politicians or bureaucrats in the future could decide that there are a lot of people abroad collecting social security, let's audit their citizenship.
My understanding when researching my own dual-citizenship is that you do not loose your American nationality when acquiring another nationality unless it was your intent to do so. Some countries require that you do this as a condition of nationalization (and some make exceptions for marriage, previous family ties, etc). So if you acquired a second nationality in a country that required this, knew that it meant giving up your US nationality, and willingly did so, you would no longer be American. Whether or you or the second country mention this to the US authorities is a totally separate matter, which can result in some interesting consequences later.
So if you need to renounce your previous citizenship as a condition of becoming, say, Columbian, then you would no longer be American (probably). Seeing as though he became Columbian through presidential decree, I'm guessing Columbia didn't make him do this. So now he is a dual-national, but from the US government's perspective he is American.