It is very common to book travel before having the relevant documentation, including Passports or Visas. For this reason, they are not required until just before travel will commence.
Book the tickets without entering any Passport or Visa information. Then, when the Passport arrives, update the reservation or provide the information at the airport during checking. Be sure to arrive extra early to do this.
Note, if for any reason the Passport or any visas are not issued, you may not be entitled to a refund as it is the travelers sole responsibility to have these before travel.
As outlined in a related question, airlines never bother checking if the passport number entered during booking/check-in is actually valid. In the worst case scenario you will be asked to wait for a minute before the flight while they update the number at the boarding gate.
So the answer is simple: enter whatever number you want. Just make sure that the person in question has their passport ready by the time they need to fly.
You typically don’t need a passport number to book a flight; you will only need the passport at check-in.
The risk is really that the passport does not come in time for the flight, as that would be your/his loss; he would miss the flight, and neither the airline nor the passport office will reimburse you in any way.
Credit:stackoverflow.com‘