If the person has mild dementia
Recent memory is lost first with dementia. So you'll need to adjust your approach accordingly.
Keep invoking the doctor. In early to moderate dementia, the person is likely to understand and respond to the doctor's prescriptions. But the problem is that they forget that this conversation ever took place. Frame your prompts each time as coming from the doctor: "Here are your briefs. Remember what the doctor said, that this is what you need to do for now."
Empathize. "I can understand why you don't like this. But I care so much about you -- I wouldn't want you to wind up with a bigger problem, so if the doctor said it, I guess we better do it, huh?"
Switch cold turkey. If the person can use underpants-style incontinence products, remove all the cloth underwear and replace them with the disposables. If the person notices the difference, or panics that the old underwear has been "stolen," casually remind, "Those are yours. They're the special underwear the doctor wants you to use." With repetition and adjustment, the person can learn to think of them as the "new normal."
