Paula,
I've been doing technical writing on and off since about 1986, although I've dropped it the last couple of years with the pressure of other work.
My experience in the field has always been from a purely freelance angle, submitting articles "on spec" for consideration, or getting a contract for a regular monthly article with a particular journal, paid per job. Rates vary considerably from one publisher to another and depending upon the type of publication. With freelance work you can still write for U.S. publishers from Britain of course -- I did some work for Gernsback (Farmingdale, NY) a few years ago.
Presumably though your cousin would be looking at getting a permanent position with a company in order to get a visa etc. I'm afraid I have no experience from that perspective, and I would imagine obtaining such a position would be much more of a challenge.