Upvote:0
You cannot get "booked" flight lists, but you can get "scheduled" flights.
Try to search Google for Destination Airport official website. They must have a schedule of departures & arrivals, look into arrivals list. But if your friend has a transit somewhere in between of journey, the arrival will list the last transit airport as origin.
For example, the website of Doha Airport will list AbuDhabi as origin for Delhi-AbuDhabi-Doha flight.
Second option, if you know the airline, look at airline's website for flights between your two original cities. Apart from all scheduled flights, it may also give you an idea of transits, if any.