Every bathroom trip starts with her getting up from the bed, so the bed is the first stop to check. Do this on a night when she is in it and sitting up the way she usually does. Measure from the floor to the top of the mattress with a tape. Then sit her in a chair you both use, measure the seat height, and see whether her feet land flat on the floor when she stands. If she has to push hard off the mattress edge, or she pauses on the way up, the bed is likely part of the problem.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
What you can usually do yourself
If the frame has adjustable legs, lower them one turn at a time and check the bed after each change. Keep all four legs at the same height so the mattress does not tilt. If you are adding a riser or a bed lift, read the maker's weight rating and check that the riser sits flat, locks in place, and does not slide when someone sits on the edge. Test it with your full weight before you leave it for her to use. A riser that shifts under a person is worse than the low bed it replaced.
What is worth hiring out
Hire it out when the change means taking the frame apart, moving a heavy mattress base, or buying an adjustable base that needs a power outlet in a place the outlet does not reach. Before anyone is booked, ask who assembles the new frame, who hauls away the old one, and what happens if the new height is still wrong. Ask whether the return or exchange is free and who pays to bring the bed back. A company that answers those questions in writing is easier to hold to the job.
Ask the clinician first
Ask her doctor or physical therapist what bed height suits her hips and knees before you change anything. That is their call to make, and it may point to a different height than the one you would pick by eye.
What to write down
Write the floor-to-mattress measurement before the change and after it. Write the seat height you matched it to, the name of the person who did the work or the part that was added, and the date you checked it under her weight. Keep the receipt or delivery paper with those numbers. If the bed height ever feels wrong to her, you will have the starting point written down.