if you know of any airline with a particularly good (or bad) reputation for providing these services, please let me know!!
I have flown with United for the past 5 years and they are always very helpful with individuals that need assistance. They make an announcement prior to general boarding of the aircraft for people needing assistance to board prior to everyone else. They also have an employee to help with any needs of the individual.
That being said, if your friend flies an American carrier, the airline must comply with the American with Disabilities Act, so the airplane will be wheelchair accessible. If your friend hasn't made the reservation, usually you can indicate that you will need assistance with boarding as well as plane changing. If your friend has already made reservations, have them call the airline ahead of time so that they will know what is needed to assist your friend.
We've had a good time traveling with American Airlines. You just need to get there a little earlier and let them know ahead of time that you need a seat as close to the front as possible, if you're not in 1st class, you can call the airline for this. Then ask to board early. They have always been very accomidating. Hope all goes well.