I looked at the altitude profile of the North Face Trail.
I would not dream of hiking it. It is steep, and at high altitude which would cause shortness of breath unless you are acclimated for a few days.
I am a couple of years older than your mother, have moderate knee pain which has almost gone way due to treatment for the originally severe knee pain. In both knees I have a torn meniscus and arthritis.
I can walk a few km with little or no discomfort now,as long as I walk on smooth surfaces - roads, smooth trails - in ordinary shoes Actually, I wear training or runners shoes....sneakers?...not sure what your term might be. They are cloth and leather, made for runners or athletes, have a fairly wide sole and heel for stability, good arch support, supplemented by custom molded insoles ("orthotics.")
www.newbalance.com/pd/ mens-new-balance-990v4/M990-V4.html?dwvar_M990-V4_color=Grey_with_Cas tlerock#color=Grey_wit h_Castlerock
If I try instead to walk on a rough trail that causes my feet and ankle to grip the ground at different angles, and have significant ups and downs making my knees flex a lot, my knees start to hurt - a lot.
Even before my knees were troublesome, a steep downhill stretch like the last 250 meters of the North Face Trail would cause my knees to be sore for a few days, and they would be very tired and sore at the end of the descent. To prevent that, I had to exercise by climbing on hills and do related in-door exercises for strengthening the knees.
The end of that trail is hard work, as is the climb at the beginning of the hike. Downhill is notoriously hard on knees.
To hike on trails that do not have smooth surfaces I wear boots with high ankles. The support allows me to deal with the irregular surface.
I did the Männlichen to Kleine Scheidegg walk in September, with boots and hiking staffs. Only a trace of pain, toward the end. This description is accurate:
<<"The hike from Männlichen to Kleine Scheidegg is a fairly easy short route with only slight ascents and descents. From Kleine Scheidegg, there is a beautiful view of the Jungfrau massif. The trail leads through grasslands where many alpine flowers can be seen in Summer. This route is perfectly suitable for families with small children and for people who want to walk in the high mountains without too much effort.">>
The route has an altitude drop of about 300 meters, spread out over 4 or 5 km. They are a few modest steeper up an down stretches. A gravel surface on the trail requires caution, and/or support from a hiking staffs, as well as grippy soles on the footwear, in order to avoid slipping on the short somewhat steeper sections with gravel surfaces.
Alpenrose suggested the walk from Lauterbrunnen to Stechelberg.
It is smooth, nearly level, and has bus stops along the way. You can see the road along the valley floor in my image of the LBV from Wengen, posted above.
The sight of the mountains, and some beautiful waterfalls along the way make it a very nice walk.
At Stechelberg, you can take the cableway up to Mürren.
PS - I have added a couple of old pictures of the Brienzer Rothorn train. It is a spectacular train ride.