I would drop the whole issue for now - you won't persuade her otherwise and may just ingrain a negative response.
She's not exactly in denial, she is no longer really able to think the situation through on a rational level.
Distract her with something pleasurable and don't yourself bring up the diagnosis or appointment. On the day just take her as if you are going out for some treat - and happen to have to pop in on the way - maybe even pretend the appointment is for you.
As for the meds - I would tell my dad it was time for his meds very matter of factly and offer tablet and water as if this were just nothing out of the ordinary. Don't ASK if he wants to take them - questions will get NO answers. At any resistance just take them away. Don't explain or argue. Don't get into whys or who said so or where they came from or even that she has never taken any pills before. Do offer them again 10-20 minutes later, again no fuss and definitely no reminder that you'd tried this before - maybe in another place eg call her into the kitchen to choose a biscuit and just happen to have the meds there too.
Check with the consultant/GP which if any meds MUST be taken and which you can safely leave if she keeps resisting. Dad's consultant said not to force the issue and start a war when peace was more important than the meds.
Just because she refuses once does not mean she will refuse next time. Though if she continues ask if they can be in liquid form and maybe hidden in a drink or crushed and hidden in food - but don't just do this as some pills should not be crushed.
