Fluid replacement would be the first port of call if the gear changes have become jerky rather than the the smooth as silk we are used to. This may not resolve the symptoms you describe. The usual fault is the torque converter cracks leading to fluid leaking into reverse gear, this would stop the car dead not the high revving, parts are available to do repairs as this box is used in Jags, VW, Ford Galaxy, and many more.
The recommended specialists are http://www.ashcroft-transmissions.co.uk/
Carvers Transmissions Ltd
Unit 2, Turner Street
M34 3EG(Road Map)
Tel: 0161 320 3400
Fax: 0161 3203433
Ask for Chris he owns the place
both can supply reconditioned units or can repair your current box.
If she is happy with the general condition of the car then a repair is far the better option than taking a risk with another used vehicle which could have any amount of problems waiting to surface.