You notice the split caulk while you are standing in Dad's bathroom, trying not to stare at the same towel that has apparently been hanging there since the invention of towels.
Dad sees you looking. He says he can recaulk the tub. Of course he can. He may also mention that he has recaulked things since before you were born, which is both possible and not quite the question.
The question is whether this particular job is a clean strip of failed caulk that the two of you can replace, or whether the gap is only the visible part of a water problem. You are not deciding whether Dad is capable. You are deciding what the tub is asking someone to do.
Start with the whole water line
Do not inspect only the ugliest inch. Follow the seam where the tub meets the wall from one end to the other. Look around the faucet, handles, soap dish, shower door track, and outside corners. Then look at the floor beside the tub and the ceiling below it, if there is one.
Press gently near the seam with your fingertips. The wall and floor should feel firm. Stop if a tile moves, the wall gives under light pressure, the flooring feels soft, or the trim is swollen. Also notice peeling paint, dark staining that returns after cleaning, a damp smell, or water marks outside the tub.
Run the shower briefly while both of you watch. Keep the spray inside the tub. Look for water escaping around the door, curtain, faucet trim, or tub edge. If water appears downstairs or inside a nearby cabinet, turn it off. That is not a caulk-first job.
A narrow split in otherwise clean, firm, dry material may be a reasonable do-it-together repair. Loose surfaces, recurring dampness, or water appearing somewhere else call for someone who can trace the route before covering the opening.
Decide whether the position belongs to Dad
Recaulking looks like a small hand job, but much of it happens bent over the tub or kneeling on a hard floor. Removing the old material can take longer than applying the new bead. The person doing it needs enough hand control to guide a scraper without slipping, enough sight to see what remains, and enough steadiness to get down and back up.
Ask Dad to show you where he would sit, kneel, or lean. Do not make him perform the whole maneuver as a test. You are looking for an ordinary setup he would actually use, not a heroic demonstration powered by irritation.
If he can work comfortably from a stable seat outside the tub, reach the full seam without twisting, and handle the tools securely, doing it together may fit. You can remove residue while he lays the new bead, or the other way around. Dad still owns the bathroom and the decision. You are simply dividing the awkward parts.
If the job requires him to climb into a wet tub, kneel when he cannot rise easily, stretch behind fixed doors, or balance with one hand while cutting with the other, hire out that part. Stubbornness is useful for many household tasks. It is less useful when paired with a sharp scraper and one sock on a tile floor.
If you do it together, finish the preparation
New caulk does not make old, loose caulk dependable. Remove the failed material without gouging the tub or wall. Clear away soap film and residue. Let the seam dry fully. Check the product label for the surfaces it can be used on and how long it must remain dry before the shower is used.
Before opening the tube, gather everything needed for the full job: a suitable removal tool, cleaning cloths, gloves if Dad uses them, a trash bag, the caulk, and a way to smooth the bead. Arrange a different place for him to wash while the tub is unavailable.
Apply one manageable section at a time. The goal is a continuous seal attached to both sides of the seam, not the fattest white rope either of you can squeeze from the tube. Wipe spills while the product instructions allow it. Leave a note on the shower handle with the time it can be used again. Dad may remember perfectly. The note is for the other fellow who wakes up and turns on the water without his glasses.
If you hire it out, name the whole job
Tell the person what you observed before asking for the work. Say whether the surface is firm, whether any tile moves, whether water has appeared outside the tub, and whether the old caulk has been patched before.
Ask what the visit includes. You want to know who removes all failed material, who cleans and dries the seam, what surfaces the new material is meant for, how long the shower must stay unused, and who handles cleanup. Ask what the worker will do if the wall, floor, or tub edge is damaged after the old caulk comes off.
If there may be a leak behind the wall or below the tub, ask whether the person can locate and repair that problem or whether you need a plumber or another repair professional. A neat new bead should not be used to hide a route that water is still traveling.
Agree with Dad about who will meet the worker and who can approve a larger repair. If the worker finds something unexpected, the answer can be to stop, take pictures, and discuss it with Dad. The open seam does not give anyone permission to redesign his bathroom before lunch.
What to do tonight
Dry the tub edge and place a towel outside the area where water has been escaping. If the wall or floor feels soft, water appears beyond the tub, or the ceiling below is damp, stop using that shower until the source is understood.
Take clear pictures of the entire seam and any staining. Put the old caulk, tools, and a replacement product on tomorrow's list, but do not seal a wet or uncertain gap just to make it look finished.
Ask Dad how he has been washing and whether anything about stepping into the tub, standing under the shower, reaching the controls, or getting out has become difficult. Listen to the answer before turning the conversation back to caulk.
When to ask the doctor
Contact Dad's doctor if the bathroom problem leads you to notice that he has stopped washing because of pain, dizziness, shortness of breath, weakness, trouble standing, or fear after a slip. Also call if he has fallen, has a new injury, or seems suddenly confused about a bathroom he knows well.
You do not need to decide what is causing the change. Tell the office exactly what happened: what Dad was trying to do, what stopped him, whether he fell or nearly fell, and what is different from his usual routine. Ask what should happen next and how soon he should be seen.
Add this line to the good-week card
The tub edge is dry and firm, and Dad can use the shower without working around the repair.