Dad has agreed that a neighbor can check on him. Good. Do not turn that small yes into an unpaid home-care arrangement with no edges.
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The useful question is not whether the neighbor is trustworthy. It is whether this particular neighbor can perform a clearly defined job in this particular house when Dad does not answer.
Start with the event you are trying to cover
Pick one trigger. Not a general instruction to keep an eye on him.
A workable trigger sounds like this: Dad normally opens the blinds before breakfast. If they are still closed when the neighbor walks the dog, the neighbor knocks, calls Dad and then calls you.
Other triggers might be an untouched newspaper, a porch light left on, a missed ride or Dad failing to appear for a routine he chose. Use something the neighbor can observe without entering the house or keeping Dad under surveillance.
Ask Dad which routine he is comfortable using. Then ask how often that routine legitimately changes. If he sleeps late twice a week, closed blinds are a poor signal. If he frequently leaves his phone in the basement, one unanswered call proves very little.
Fit the job to the neighbor
A neighbor who is usually home can notice a visual signal. A neighbor who leaves early may be better as an emergency key holder. Someone with limited mobility should not be assigned a basement search, an icy walkway or the job of lifting Dad.
Ask the neighbor four plain questions:
Can you reliably notice the agreed signal? Are you willing to knock and call? Are you willing to enter if Dad has already given permission and the situation calls for it? Are you willing to call emergency services even if Dad may be angry afterward?
A no to any one of those questions is not disloyalty. It tells you where the assignment ends.
Walk the actual route
Do one daylight walk-through with Dad present. Start where the neighbor would start.
Check which door the neighbor should use, whether the storm door sticks, whether a locked interior door blocks the route and whether pets can escape when the door opens. Identify the rooms the neighbor may enter and the rooms that remain private. If the electrical panel, water shutoff or medication list matters to the plan, verify that the neighbor can find it without hunting through drawers.
If a key is part of the arrangement, test that exact key in the exact lock. Do not assume a garage remote solves access. Power can fail, an interior door may be locked and the remote may be inside the wrong car. For a broader access setup, use the site's Start Here page to separate an ordinary check-in from an emergency response.
Write the stop line
The neighbor needs to know when to stop being a neighbor and call for trained help.
Use observable conditions. Dad calls out that he cannot get up. The neighbor sees him on the floor through a window. There is smoke, water running, a strong gas odor or an alarm sounding. Dad answers but cannot open the door and sounds seriously confused or distressed. The neighbor enters with permission and finds an unsafe scene.
The instruction should be simple: call emergency services, give the address, describe what you can see or hear and follow the dispatcher's directions. Do not ask the neighbor to lift Dad, force a door, diagnose him or decide whether an injury is serious.
Missouri law provides a formal route when access to an eligible adult is barred and there is a substantial likelihood of serious physical harm. A peace officer may seek a court warrant to enter and remove the adult for protective services. That is an emergency authority with legal conditions, not a standing permission for family or neighbors to break in. See Missouri Revised Statutes section 192.2465 in Sources.
Do not make the neighbor Dad's boss
Dad's agreement to a check-in does not hand the neighbor control of the house. Write down what Dad has actually authorized: knocking, calling, using a key after a stated trigger, entering specified rooms and contacting you or emergency services.
A power of attorney does not automatically erase Dad's right to refuse. Missouri law specifically says an attorney in fact cannot be given authority to require the principal, against his will, to act or refrain from acting. The document's wording and the circumstances still matter. See Missouri Revised Statutes section 404.710 in Sources.
If Dad opens the door, appears able to make his own decision and tells the neighbor to leave, the ordinary plan ends there. The neighbor can report what was observed. Immediate danger belongs with emergency services. A continuing pattern belongs with the family, Dad and the appropriate local service, not with a neighbor improvising authority.
Build one backup, not a committee
The plan needs a second contact for the times you are driving, asleep or out of town. Give the neighbor your number and one backup number in order. Decide how long to wait before trying the backup. For a possible emergency, the instruction is to call emergency services first, not work through a family phone tree.
Keep the information narrow. The neighbor usually needs Dad's full name, address, phone number, the agreed trigger, the entry instruction and the contact order. A diagnosis list, financial information and family history do not improve the knock on the door.
If the arrangement needs more than a neighbor can reasonably carry, compare it with the options on Local Help in St. Louis. The neighbor can remain the person who notices. Someone else can own the recurring work.
Test it while nothing is wrong
Run one announced test. Dad skips the normal signal. The neighbor notices, knocks, calls and follows the contact order. Confirm that the phone numbers work, the key opens the door and everyone understands the stopping point.
If the test requires repeated explanations, the plan is too large. Reduce it to one signal, one permitted response and one escalation path.
Your next move tonight: Text Dad and the neighbor a three-line draft containing the trigger, the neighbor's exact job and the point at which the neighbor calls emergency services. Ask each of them to reply with what they believe they are agreeing to.