She walks the same six stops in the dark, often in the same slippers, and the shoe is part of the route. A slipper that looks comfortable in a store can slide on bathroom tile or catch on the edge of a bath mat. You do not need a brand to judge a night slipper. You need to check a few physical features and test the pair on her floor.
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Check the back and the heel
Look for a closed back or a firm heel counter, the stiff piece at the back of the shoe that holds the heel in place. A slipper with a soft, folding back tends to slide off the foot when she steps, and a loose heel is harder to control on the way back from the bathroom. Press the back of the shoe with your thumb. If it folds easily, it is probably the wrong pick for the night walk.
Look at the sole
Turn the slipper over and check the tread. A tread with some texture grips better on a bedroom rug than a smooth leather or fabric bottom, but a very thick sole can catch on a threshold or a mat edge. Walk her across a bare hard floor in the slipper and listen for slipping. Do the same on the bathroom tile if you can.
Fit it with her usual socks
Try the slipper on with the socks she actually wears at night. A shoe that fits in a store can feel too loose at home when the socks are thicker. The toe area should not push her toes forward, and the slipper should not be so tight that she has to force her foot in.
Compare two pairs on the route
If you are choosing between two pairs, have her walk stop one to stop six in each, without the lamp if she usually goes without it, and then with it. Note which pair she stands up in easily, which one slides, and which one she kicks off. A returnable pair is worth more than a slightly better one you cannot exchange. If she has balance or foot concerns, ask her physician or therapist before choosing, and start with the reading path at Start here: a reading path.