Thank you for asking this question. It is so important to be safe with folks and to make sure that they are getting appropriate care especially when they are on medications. Some situations are beyond the scope of the Home Herbalist and Community Herbalist Program.
For the therapeutics assignment, we mean for you to create a theoretical outline and protocol. If you actually would like to use the herbs that you come up with, for your dad, we strongly recommend that he consults with his doctor or a herbal practitioner first to make sure that he is getting the best and safest care he can be.
There are free student clinic consults that he could partake in through Pacific Rim College in person and online. https://www.pacificrimcollege.com/clinical-services/
Here’s a resource about Candesartan that is worth reading to understand where and how herbs may interact. https://www.accessdata.fda.gov/drugsatfda_docs/label/2006/020838s026lbl.pdf
It discusses how it works and is metabolised along with some side effects. You are right, diuretics should be avoided (dandelion leaf), and caution taken with sedatives, or antispasmodics due to hypotension, also stimulants may need to be avoided.
I look forward to seeing what you come up with. Or you may choose another topic if you feel overwhelmed by this of course!