Dr. Soe Maunglay started working for Dr. Farid Fata at his private cancer practice in 2012 and realized something bizarre was going on.
"I discovered a patient receiving treatment without actual diagnosis of cancer," Maunglay told CBS News correspondent Dean Reynolds.
Maunglay reviewed the patient's record and saw "there was nothing really to support that the patient has active cancer."
He asked the patient "in a very sarcastic way" who diagnosed her disease because he knew who had.
"I was enraged so I asked her the question," Maunglay said.
After the discovery, Maunglay alerted the practice manager during the summer of 2013 who then contacted the feds. Fata was in handcuffs less than a week later.
When Maunglay found out Fata had been arrested, he said he felt "very satisfied that at least this has stopped."
"I think he's guilty of the most cruel thing that a human being can do to another human being," Maunglay said.
Fata was back in a Detroit courthouse again on Wednesday and have to face more victims of his health care fraud. Prosecutors say cancer Fata gave unnecessary chemotherapy to patients, and some of Fata's 553 victims were never sick. Fata pleaded guilty to 23 counts of health care fraud, conspiracy and money laundering.
Former federal prosecutor and Wayne State University law profession Peter Henning said Fata was "looking people in the eye and telling them you need to have this treatment when it was completely unnecessary."
He said he has never expected that out of a doctor.
"Who's more trusted then your own doctor, especially when you get a cancer diagnosis?" Henning said.
Former patient Teddy Howard went through rounds of chemotherapy after Fata's diagnosis. Howard doesn't have cancer and now needs a lifetime of medicine to stay healthy.
"What really makes me angry is the fact that he lied. He knew he was lying, he gave the drugs to me anyway and I had no knowledge of it and now my life is turned upside down," Howard said. "I can't do anything about it. I don't know how long I'm going to live."