Man, do I relate to this situation. Only it's my spouse, not my dad (my dad is totally anti-drug, and you can't even have a rational convo with him on the subject.
My spouse is disabled and in a lot of pain every day. She knows about all my shit....the poppy tea, the drugs, the works and is not happy about it, but doesn't push the issue because I am basically a fully functional addict and I have convinced her that drug legalization is our constitutional right and that drug laws have caused a lot of harm. She takes tramadol and has a small stash of morphine sulfate for emergencies, but she doesn't touch the stuff. She'll also do mary jane, when it comes her way, either via edibles or hemp oil.
So, she is okay with marijuana and I've even ordered her tramadol online but, she won't take anything else I offer, even half a 10mg valium for her severe muscle pain.
It's so goddam frustrating, because I know I could make her feel better with the stash I have, but it would be an EXTREME violation of her trust if I ever spiked her cocktail or did something like that without her consent. So, it's not an option here and I think the same principal should apply in the OP's situation.
Can you imagine how angry your dad would be if he somehow found out? You would lose his trust forever.