I have a pair of £50 boots which beat any £100 boots I've ever wore, simply because they were the perfect fit, price doesn't mean everything.
Depends what sort of walking she's doing, if it's normal walking round the town sort of thing you can get away with trainer style boots, if it's more country side/hills/small mountains, get something with a bit of ankle support. In case I didn't make it obvious earlier, make sure you get them fitted, any respectable outdoor shop will help you, Blacks/Millets are great "budget" shops, if you have one local, go to Field & Trek, service is second to none (except in Leicester :/)
As mentioned get a good pair of socks, these really do make a massive difference, climbed Ben Nevis last year without good socks, my feet suffered. My boots a year older and very worn but with a good pair of socks made it surprisingly easier on my feet.
Cheaper boots do wear out faster, something to take into consideration. If you have a Dr Marten shop local, I'd definitely suggest checking out their lifetime range; free repairs/new shoes when they wear out