A lock job looks like a simple choice between fixing the old lock and buying a new one. The real decision is who holds each key, and how a neighbor, a helper or a responder gets through the door if she cannot get to it. Settle that first, and the hardware choice gets much easier.
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.
Count the keys that already exist
Keys multiply in a house that has been lived in for decades. Before you call anyone, list every key you know about: hers, the spare on a hook or under a pot, the set given to a previous handyman, the one a former neighbor borrowed. Ask her where each one is. Some you will never find. Assume those keys still work unless the lock is changed.
That is the case for rekeying. A rekey changes the internal pins so the old keys stop working while the lock body and the look of the door stay the same. Replacing the lock is the stronger step when you cannot account for the old keys, when the lock is damaged, or when the door is metal or has more than one locking point. A locksmith can tell you which applies to her door.
Choose the spare-key holder
Pick one or two people who will hold a spare and say so out loud. A neighbor who is home most days is often a better choice than a relative three hours away. A local helper who already visits can hold a key if she agrees and the arrangement is written down. A key lockbox is an option some families use, but it puts a code in circulation and can be found by people you did not intend, so weigh that before you choose it.
Tell her who has each key. She should know who is allowed in and who to call if she is locked out. Do not leave a key under a mat or in a planter. That is the first place a stranger looks.
Hire or do it yourself
A rekey kit can be a reasonable job for a visit if the lock is a standard keyed deadbolt, you are comfortable with the instructions, and she is home to test every key afterward. Keep the old parts, label the new keys, and test the lock from both sides.
Hire a locksmith if the door is metal, the lock has multiple points, the cylinder is hard to reach, or you cannot be sure the new key works before you leave. Ask for the state license number and proof of general liability insurance, then check the license with the state before the appointment. Ask whether they will rekey or replace, whether every key is cut and counted, and whether they will test with her present. Local people who can help are listed on the Local Help for a St. Louis Parent page.
Plan for an emergency entry
Think through a bad morning. If she has fallen inside the house and cannot reach the door, can a neighbor get in quickly? Can a responder? Write down where the spare is kept, who to call first, and what the neighbor needs to know. A plan that works only when you are on the phone is not a plan.
What to do tonight
Write down every door and every key you know about. Ask her which keys she has and where the others are. Call the neighbor you trust most and ask whether they would hold a spare. Then decide whether you will rekey, replace, or ask a locksmith for an estimate, and call two companies with the same description of the door.
What to leave with the doctor
Bring up grip and hand use at her next visit if she has trouble holding or turning a small key, or trouble getting a key into a lock in cold weather. Ask the doctor whether any of that should shape the choice of lock or the way she gets in and out. The lock plan should fit what she can actually use.