Andrew Ridgeley took to social media on Thursday to criticise the show, titled The Last Days of George Michael.
The former Wham! star said the channel had been "insensitive, contemptuous and reprehensible" and should have waited until after his friend's funeral.
But Channel 5 said it was "a measured account" of Michael's life and death.
"George Michael was a high-profile public figure and there has been legitimate public interest in the circumstances surrounding his death," the broadcaster said in a statement.
It said the documentary, which aired at 21:00 GMT on Thursday, had been "based on contributions from those who knew him and reported on him over many decades".
Ridgeley, who paid an emotional tribute to Michael at last month's Brit Awards, had called the programme a "piece of voyeurism" that should only have been aired "after a respectful period".
It was confirmed earlier this month that Michael - who was born Georgios Kyriacos Panayiotou - died on Christmas Day as a result of heart and liver disease.
Fans of the late singer called Channel 5's programme "pathetic", "disrespectful" and "utter trash" on Twitter.
One who complained directly to the broadcaster received a reply saying it was not the show's intention "to upset or cause any offence".
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Channel 5 has defended broadcasting a documentary about George Michael after the singer's former bandmate dubbed it "sensationalist and mucky".