How I can get a urine sample from mom while she’s using a diaper?

Can your mom safely sit on the toilet? If so, there is a plastic bowl type contraption they refer to as a “hat” that you can buy at the drug store. Purchase 2 of them and a lidded cup. Place them back to back under the seat of the toilet, then lower the seat onto them. Having 2 placed this way will keep Mom from missing one of them. You don’t want to go through all the trouble to get her seated, only to hear the pee going right into the water. Ugh. As soon as she sit, clean her lady parts good with a steri-wipe and then pour the pee into the cup and hustle it right off to the doctor. If all this is impossible, a trip to the doctor or urgent care may be needed where they will inset a tiny catheter and collect the urine that way.

You could set her on the toilet with a "hat" below her if she sometimes pees that way, or you need to ask her doctor about whether they will use a catheter at the dr's office.