Police said that he found the BMW with the engine still running, and saw Zainab walking near the car, about to faint. He then discovered the other cyclist, who had passed him earlier, dead on the road.
After alerting firefighters at about 5 p.m., the witness broke the driver's window and saw the bodies inside, one of which covered little Zeena. The cyclist then placed Zainab in recovery position, the BBC reported, adding that police say he effectively saved her life.
Authorities removed her at around midnight, and she is being treated by psychiatrists.
Zainab is in critical condition in the hospital in Grenoble with a fractured skull.
Chevaline Mayor Didier Berthollet told the BBC News that most local people had already been questioned by police, describing the situation as "surreal."
"Things are very precious," Barthollet said, adding that the deaths reminded him of a similar death of another family seven years ago not too far away.