Hi there!
I have a pretty severe form of auto-immune arthritis, and while I'm normally not wheelchair bound (a cane on bad days), when I go to WDW I always get an
ECV or a wheelchair.
I can tell you from experience that even if your friend does manage to walk through a park all day, she WILL pay for it that night and the next day! You just can't underestimate the toll it will take on your body.
I recommend the ECV for Epcot, for sure, because its the most amount of walking. If you have somebody to push you, I recommend a wheelchair for MK, because there are so many more (and younger) bodies that stop in front of you or don't see you, also MK has so many rides that are difficult to maneuver in a ECV that sometimes a wheelchair will be easier. Buzz Lightyear, Space Mountain, and POC come to mind.
That said, please let her know that just because she gets a form of ECV or wheelchair it does NOT mean she's chairbound for the whole day! Most of the time I park outside a shop or restaurant and walk through, then get back in my chair when I'm done. It is hard to navigate in the shop aisles, and I hate having to tell DH when to push me where, so walking is easier there.
Hope this helps! Let me know if you need any more tips for WDW with arthritis, LOL!