There's really no way to know so it's silly to ask for a source. The only crime statistic that's fairly reliable (at least in developed countries) is murder because it's hard to hide a body. Lesser crimes are heavily under reported but no one knows by how much, and the reporting rates vary widely by location and time. In many places the police actively discourage victims from reporting minor crimes because it creates more work for them and looks bad in their statistics.
The Crime Survey for England and Wales (formerly the British Crime Survey) should provide some insight, since it's based on interviewing a random sample of the general public.
It suggests that around a third of violent crimes are not recorded by police - but, contrary to the gp's assertion, that proportion doesn't seem to have changed significantly since the BCS began in 1982.
Many say the opposite. Crimes are more often reported especially if suspects have a „specific skin color“.