Unfortunately, the default answer from someone with dementia is almost always "no". Trying to reason with them just makes it worse - logic is not your friend when dealing with dementia.
Usually they answer "no" because they dont understand what they are being asked to do, they dont understand why they have to do it, or they dont remember how to do it, so its much easier to just refuse. So you will have to use your powers of persuasion. If its something like getting them to wash or change their clothes then there are various techniques which might help (like "accidentally" tipping cold coffee down them). If its something like getting them to go to the doctors it much easier not to tell them in advance and just treat it like a "done deal", or else get them out with the promise of tea and cake (often works wonders).
If you could elaborate a bit about what she is refusing to do, Im sure members will come up with some ploys that have worked.