The bed edge is the first stop on her night walk, and a new mattress changes it. A thicker top layer can put her seat higher or lower than before, and that changes how she pushes up and how far she has to reach for the floor. Before you sign anything, treat the mattress paperwork as part of the route, not just a shopping step.
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Get the finished height in writing
Ask the seller for the height from the floor to the top of the finished mattress, and ask whether that figure assumes the foundation or box spring you actually have. A number for a showroom display on a different base is not the number that matters. Write down the bed frame you will use with it, and ask them to note that frame on the order.
Read the return and the warranty before the delivery
Ask what the trial period is, whether returns are refunded in full, who pays to haul away the old mattress, and whether any fee applies if the mattress is opened or used. Then ask what the warranty covers. Look for whether it covers sagging, edge collapse, or both, who handles a claim, and whether a claim needs the original packaging. Ask for each answer in the written order, not only on the phone.
Check the edge the same night it arrives
When the mattress is in place, have her sit on the edge and stand up three times with the lamp on. Watch whether the edge gives under her, and whether she has to push harder than she did on the old bed. If she does, call the seller before the trial period runs out. If she feels unsteady or dizzy when she sits up, that is a question for her physician, not a mattress setting to adjust.
Write down what you agreed
Keep the order, the height figure, the return date window, the warranty terms, and the name of the person you spoke with. Put them in one folder with the bed frame model. If anything changes later, you will have the paper to point to. For how we check the paperwork we read for this desk, see How we write this desk.