Now that I think about it - we've never been to WDW
without taking someone else along. Huh, maybe we should try it by ourselves sometime. We probably wouldn't know how to act. We do always stay at the same hotel - it makes the logistics easier.
Most important: We always set a clear expection out of the gate: "No one should feel obligated to do what we're doing. It's completely OK with us for you to go off and do your own thing. And vice versa."
Then we plan the vacation we want to go on. We share all the information since, if our friends or family might want to come along we might need ressies for more folks. Usually, we'll try and find one or two days where we all do something together - in that case we will talk about what's most important to everyone and plan for it. Sometimes it's a dinner show. Sometimes a day in the park. There's always something that everyone can get into. Perhaps for your friend it'll be one of the water parks - loads of pool sitting to do there.
We've discovered that as we start to share information, people start to get educated about what's available, and start to have opinions. Let's face it - it's pretty overwhelming for the first timer. As they start to get an idea of what they want their vacation to be - and maybe it
is hanging out by the pool. Then we start to see how / if that can be worked into what we want our vacation to be. Perhaps we schedule in an afternoon swim where we meet up with our sunbathing friends.
The Key is to talk, talk, talk. It makes me crazy when folks say "Oh, just plan it and we'll do what you do." My usual response is something like "I can't meet expectations that I don't know about." or "If you don't provide input / participate now, you don't get to complain later." Then I quite stubbornly continue to give them information - the last thing I want on my vacation is a bunch of complainers.

Eventually, they catch on.
If your friend doesn't start to clue in as you talk about it then you might have her tune into the Travel Channel or one of the other programs that gives information about WDW. Even the WDW planning DVD might work - though I've never seen it.
I think if you start with the premise - that it's OK for everyone to have the vacation they want. And it's OK for us not to spend every minute of the day together. Then it becomes a lot easier to make everyone happy. Since, in the end,
the only happiness you are responsible for is your own.