You can't at least, not in the capacity of a friend and not for any length of time.
You can provide a letter or even an Affidavit of Support to back up a visitor visa request - but since your friend is from the UK he doesn't really need a visitor's visa for a short visa; he can travel under the visa waiver p[program/ESTA for a brief visit.
Anything more than that, are are really only two types of "sponsors" (technically, "Petitioners"): a close family member, and an employer.
If your friend has no close family members here (and even if he does but they are someone other than a spouse, parent or over-21 child US citizen), the only practical solution would be an employer-sponsored case based on your friend's work experience/educational background.
You/your friend need to speak with a lawyer, and supply his resume, to see what might be possible.Ask a similar question
Foreign-born persons cannot immigrate to the U.S. through friends. They can immigrate their relatives, employment, asylum, the visa lottery, etc.
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