I have a friend in Belgium that wants to move here to the US. She has a passport. She does work at the UN there, but is not in a professional field. And where she is moving to, there are no (UN) offices close to work at. So she will change fields. We are trying to figure out how do do this. And what is needed after she gets here so she can stay.
The following are pretty much only the following ways your friend could move/immigrate to the US:
1) Marry an American citizen or green card holder (permanent resident). Requires affidavid of support from someone in the US.
2) Get a company to sponsor a work visa, e.g. an H-1B. There are a limited number of them, most being used by Microsoft, Google and other big companies to import foreign engineers/programmers. These visas are time limited. They can be converted into green card/permanent residency if the employer sponsors that as well. Unless she is a very skilled worker with unique skills, this is unlikely.
3) Get a transfer within the UN (G visa) or within another company where she has been working for at least one year (L visa).
4) Win the right to apply for a gren card in the diversity visa lottery (a.k.a. green card lottery). It should open again in October 2016.
There are of course a number of very specialized visas, you can find a good overview here: https://en.wikipedia.org/wiki/Visa_policy_of_the_United_States#Classes_of_nonimmigrant_visas About green card: https://www.uscis.gov/greencard
For most people the only ways to move to the US are the same: marriage, work transfer, sponsoring by a company or winning the green card lottery. There are also investment visas, but I have heard rumors that due to mis-use of them, they may go away soon. On top of that you have a number of much more specialized visa types, for artists, professors, au-pair, etc. But for most I woudl say that the ones listed above are the ones available.