If you and your friend cannot sit together because the plane tickets are separated, then you can log in “your booking” on the airlines’ website. You can then select one empty seat for you and your friend can select the next seat.
If the airlines website is not available or you do not want to pay to select seats, then you have to wait until the online check-in opens (24 or 48 hours before departure). Then you and your friend can select two seats together respectively.
If you are too late and your online check-in is closed, then you have to go to the airport for check-in. You can select the seats in self check-in computer or ask airlines check-in staff to change seats for you.
If they cannot change seats, then you need to go to the gate staff to check whether they are two seats together available.
If the gate staff cannot change seats because it is full, it is not the end of the world. Do not panic and do not argue with gate staff as there is nothing they can do for you. But they may promise you that they will ask flight attendant to help you to change seats.
If all the above fails, you might have one last chance to change seats. After you have boarded the plane, you have to wait for seat-taken passenger to come to the seat. Flight attendant will ask that passenger whether he/she be happy to move to another seat. Then you can sit with your friend together. Do not forget to say thank you to them.
If that passenger said no, do not fight and move on. Accept that. It is only a flight of several hours!
Inflight Disability Information 