There are loads in Pembrokeshire and the national park authority have beach wheelchairs in the most accessible areas.
Newgale and Whitesands are both extremely wheelchair-friendly. Newgale has walkways at multiple points over the shale bank and toilets in the car park. Whitesands would be my choice though as it has the beach wheelchairs available from the bottom of the car park, staff to help out if you do get into bother and accessible toilets too. The disabled parking bays are right on the beachfront so even if she doesn’t feel up to leaving the car she can enjoy the view, sounds and smells. The sand is also very firm most of the time hence why it’s a favourite spot for kite landboarders, no sudden soft spots.