MacKerricher State Park has a nearly-flat boardwalk that is about a mile long near the cliffs. A rollator-style walker with a seat would be better than a regular walker for this path. Then she could rest as needed. There are resting benches, but they are fairly far apart.
I found this info on the McK SP web page: "A beach wheelchair, free of charge, is available upon request for use at California State Park beaches in the Mendocino coast area. Call (707) 937-5721 at least 7 days in advance to reserve the beach wheelchair. Please note the wheelchair does not fold up and you will need a truck or SUV to transport the wheelchair from the kiosk or Visitor Center to the beach."
I use a rollator or walk with a cane myself and have a pretty limited range these days, but I still manage to have some very enjoyable trips. Last June I did the coast southbound from Newport, Oregon, to Bodega Bay.
Googling around for accessible Northern California led to a bunch of sites such as
Accessible trails in Sonoma County. This website reminded me of how much I enjoyed visiting Fort Ross and being able to view it from an area that is off-limits to those without a disabled parking pass. Walking in from the regular parking area would have been impossible for me.
If you and your mother enjoy scenery as seen from your parked vehicle or on very short walks, explore harbors and boat launch areas. Bodega Bay and Bodega Head are good for scenery and people-watching.
Does she have a disabled parking placard? If yes, take that along. You can use it as long as she is in the vehicle; she doesn't have to be the driver. Some of the disabled parking spots have pretty good views. California State Parks offer a disabled discount pass, good for half off camping or day use. Info
here. Both of these passes take some time to process and require a doctor's signature.
Have a great trip!