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He has several options, of which marriage to your friend is best one. There are several different ways to come to USA, and of course they may arbitrarily be divided into:
1) Visa lottery
2) Education (mostly non-immigrant but hold strong possibilities for receiving a greencard based off job offer post-graduation in specific fields)
The visa lottery is a program where individuals are randomly picked by USCIS. The chances of winning this "greencard lottery" (as it is known) are slim.
Education allows an individual to come and study in the USA for a period of time, and sometimes, the individual may be able to receive employment by a US sponsor employer, although there are no guarantees.
Employment is generally considered a painstaking process and unless you have a technical degree or medical degree - along with a job offer - there is really little possibility of actually getting a visa based on employment. The bar is set very high.
Marriage is the "easiest" assuming that the marriage itself is genuine. USCIS will review the petition for bona fide relationship. See HERE on how to file.
Asylum is complicated but if he lives in the UK, then this is unlikely to be granted.
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