After Carson's 2008 appeal, the friend, Pittsburgh dentist Alfonso Costa, avoided any prison time and instead received three years of probation, according to federal court records. Costa had pleaded guilty to one count of health care fraud, which carries a maximum 10-year prison sentence. Costa confessed to billing dozens of patients for tens of thousands of dollars in procedures that were not performed, the records show.
His advocacy for his friend contrasts with his statements in his 2013 book, "America the Beautiful." In the book, Carson calls for people convicted of health care fraud to face "no less than ten years in prison, and loss of all one's personal possessions."
Carson and Costa remain friends and continue to hold "business investments together," Carson campaign spokesman Doug Watts told the Associated Press, which first reported on Carson's relationship with Costa. CNN then independently reviewed the public records.
In a statement to CNN, Carson emphasized Thursday that Costa is his "very best friend" and explained why he appealed on his behalf for leniency in the sentencing process.
"Al Costa is my best friend. Al Costa is my very best friend. I know his heart. I am proud to call him my friend. I have always and will continue to stand by him. That is what real friends do!"
The campaign declined to comment further on the issue, including why Carson has called for lengthier prison sentences for others convicted of health care fraud.
CNN has also reached out to Costa for comment.
The 2007 property deed for two companies owned by Carson and his wife --- BenCan LLC. and INBS LLC. -- list a Pittsburgh office building that is also the address for Costa's real estate company, Costa Land Co. Carson and Costa's companies also list the same office suite number. Carson and his wife live outside of Baltimore, Maryland.
Carson reported a combined annual income of between $200,000 and $2 million for both companies, which are each valued at over $1 million, according to Carson's financial disclosure filed with the Federal Election Commission.
Costa's wife also currently serves on the board of directors of the Carson Scholars Fund, a charitable organization, and Costa continues to serve as the president of that organization's Pittsburgh chapter, according to a chapter newsletter dated September 2015.
In his letter to the federal judge adjudicating his friend's case, Carson wrote that he and Costa became friends "because we discovered that we were so much alike and shared the same values and principles that govern our lives."
"I obviously get to meet and know many wonderful people, but there are none that are closer to me than Dr. Costa," Carson wrote in the letter, which is publicly available in the federal court records system.
"I could literally trust him with all my earthy possessions and rest assured that I would get them all back with interest. To sum it up, next to my wife of 32 years, there is no one on this planet that I trust more than Al Costa," Carson added.
Carson's letter was one of 40 written to the federal judge vouching for Costa's character.