Sam Dempster says the actor called an ambulance and waited with him after he collapsed on April 27.
On his recovery blog, Dempster thanked medical staff and "my new found favorite celebrity Dustin Hoffman who called 999 and waited by my side until paramedics arrived."
The Sun newspaper on Tuesday quoted paramedic Martin Macarthur as saying Hoffman told medics, "Great job, guys" after they resuscitated Dempster with a defibrillator.
The London Ambulance Service confirmed the incident but not Hoffman's role in it.
Representatives of the "Rain Man" star, who has a home in London, did not immediately respond to requests for comment.