What about just using a metal straw?
I'd just buy a breathing trainer with adjustable resistance (start low, set a time goal (e.g. 15 minutes once a day), and increase it one level every session until you fail, then move back one, do a few sessions, then try to move up, repeat).
Lowering Blood Pressure in 5 Minutes https://www.youtube.com/watch?v=3KueabH8B8M
What your father is doing is known as Straw Breathing, you inhale normally, then exhale via a drinking straw. Repeat for five minutes.
We're discussing bi-direction breathing training, wherein resistive inhaling is a significant part of the exercise regimen and one not offered in [easier] Straw Breathing.
If you have a study that compares Straw Breathing to using a Breathing Trainer it would be a welcome addition to the discussion. But currently straw breathing is primarily aimed at bringing people from below back to normal lung function, whereas breathing training is meant to improve lung function above normal.
As to using "a narrow straw," no, because these resist both your inhale AND exhale, and while it is easy to exhale through a narrow straw trying to inhale just causes the straw to collapse (or you won't get enough air and will have to stop/pass out). They're designed to resist but not limit.
After having tried a breathing trainer, it definitely feels like real exercise even just at your desk, and I cannot imagine doing it for 15-minute sessions via a narrow straw.