Mainline airlines* offer complimentary wheelchair assistance, usually supplied by airport contractors. When your friends make their reservation, they should call the airline and ask for wheelchair assistance. Generally the passenger(s) will need to get their own selves to the check in counter, and at that point a wheelchair is summoned, with a pusher, who is a contract employee.

Different airports have different policies about whether the pusher is required or not - at DEN, you can wander off with the airport's wheelchair, but you have to leave a driver's license as security (so of course you have to go back and get it before departing).

Tipping the pusher is nice, but not required - back when I used the service, I generally tipped $5.

At security, the passenger will be directed to the area where people with mobility disabilites are hand-searched. The passenger will be asked if he/she can stand to be searched. At the gate, the passenger will be pre-boarded - taken in the wheelchair to the end of the jetway, but will then be expected to stand and walk into the plane. The passenger's personal mobility device (cane, walker, etc) will be stowed (ie, you can't keep it with you at your seat, unless you're really good at surreptiously sliding a cane between your seat and the wall without anyone noticing).

Towards the end of the flight, the passenger should remind a flight attendant that he/she needs wheelchair assistance at the destination. It is polite to remain in your seat until everyone else has disembarked, rather than blocking others' exit from the plane.

If there is any trouble getting a wheelchair, the best thing to do is remain on the plane if possible, as the crew cannot leave while any passengers remain on board, so they're motivated to help you get your wheelchair.

Generally the pusher will take the passenger from the plane to baggage claim, then to whatever transportation is appropriate.

*Bets are off with budget airlines, especially Ryanair, which does charge for the use of wheelchairs and seems to specialize in bad publicity incidents involving wheelchair assistance.