I'll put you in context. I am from Europe and I live there. Because of my work, I usually travel to the US in order to attend scientific conferences. So, when you repeatedly participate in those meetings you meet people who work on similar topics and you cite their publications in your work.
An acquaintance met in a conference, is residing in the US and he is trying to obtain permanent residency under the classification of National Interest Waiver (NIW). He has asked me for a recommendation letter, assuming that his work was useful for me (I cited the work).
I've lived all my life in Europe and I had no idea of that kind of process. I would like to know what are the consequences of helping him. How can it affect my future relations with US (travel, internships, projects, work...)?
If I agreed, he would send me a draft written by his lawyer.
Do not hesitate to ask for more information, it is all I know, though.