Based on the following, I suspect the passenger paid the fee directly to the Airport Service Provider. It would indeed be unreasonable to expect Ryianair to pay the change for the passenger.
Wheelchair passengers:
Ryanair provides full assistance to wheelchair passengers free of charge. Up to four wheelchair passengers are permitted on any one flight. This also applies to group bookings. It is imperative that such passengers travelling in their own wheelchair advise of same, at the time of reservation as not to do so, due to the above limitation, may result in Ryanair being unable to accept the passenger for the flight reserved. In order to qualify for assistance, the passenger must be travelling in his or her own wheelchair. Passengers requesting assistance, who do not have their own wheelchair, will be directed to the wheelchair service provider at the relevant airport (see destinations link for airport information numbers). These companies will offer specialised assistance for a nominal fee which is payable directly by the passenger to the handling company.
http://www.ryanair.ie/