He'll even keep reusing the Depends. He'll take off the wet underpants, put on clean underpants, and put same Depends back on. My mom is washing 9 pairs of underpants most days. Their washing machine is downstairs and going up and down the stairs is getting difficult for her at 85 years of age. It seems like the cost of Depends is his issue. They still live in their own house and don't know exactly what she says, but my mom tells me they have argued about it many times and she doesn't know what else to say. When he first started to wear Depends, it was because he couldn't get to toilet in time. Now he is losing (or has lost) the feeling that he needs to use the toilet. So far he is able to clean himself up.
That same thought process may be going on with your dad. I love their frugality, but there are limits! So hide his tighty whities and tell mom to put her foot down about reusing the Depends. Good luck, it's not easy!
Also, I would tell her that soooo many people are wearing them now. This seemed to comfort her knowing that it is very common. She had a good sense of humor; so when appropriate we would joke about her new, sexy underwear.
This made her laugh and think it also helped with the dignity issues which are so important. Wishing you luck and take care.
I discovered my dad reusing Depends too. It was a cost thing for him. So whenever he wasn't looking I'd steal into his bathroom with a bag and bundle them up for the trash. Eventually he got the message.