Friends said he fell ill two weeks ago and was rushed to Derriford Hospital in Plymouth to receive care from specialist medics.
A relative told a Sunday newspaper: "The family were concerned. He had a pre-existing illness, then got seriously ill. Suddenly it got very critical and he was rushed to hospital.
It was pretty serious but hopefully we are all OK now.
"He is recovering and has been moved to Exeter."
The ex-Army captain and Gulf War veteran had a five-year affair with Diana while she was married to Prince Charles.
The pair met at a party in London in 1986 when she was aged 25. He was 28.
Three years later - in a now iconic image at the height of their affair - she was pictured presenting the Life Guards officer with a polo trophy.
The couple were forced apart when he was sent to Iraq in the 1991 Gulf War and split after their affair was made public.
Hewitt now lives with his widowed mother Shirley in a two-bedroom flat in Exeter.