There are only pay phones available to the public in the terminal. You should probably have her call the operator and get a collect call. Phone cards are another option - but they can be confusing for elderly people.
Please remind her to put a 1 before the area code before dialing. There are money changing facilities at the airport, and she could probably get some quarters from them. I think its a Thomas Cook booth that is located in baggage claim.
Mummee phone home!