I think your manners in this scenario were perfectly correct.

Speaking as someone who also has an artificial joint, I think it is in fact plausible that she really did need to sit down at one point, and stand at another. That's how having an artificial hip has differed from the just-plain-arthritis preceding it - instead of needing to rest all the time, I now need to maintain an arcane mix of rest and movement. Otherwise, my joints get stiff.