The thought also occurred to me as I have a similar problem as the lady. If I know I'm going to be "captive" for awhile I make sure I make a trip to the loo whether or not I feel an urgency to go. Only someone who never flies would not know that during takeoff and landing there is an extended period where the seat belt sign is on. Go before you get on the plane and again when you're about 20-30 minutes from landing. Passengers these days have the attitude that whatever I want to do and when I want to do it, it is my Constitutional right to do on my schedule.
That being said, I don't think the f/as handled it particularly well. If I see a passenger out of their seat when the sign is on, I am supposed to say to them "The seat belt sign is on." We are there to inform, not enforce. If the passenger says they really have to go (as this woman evidently did), we repeat that the seat belt sign is on, but we never say that you can't use the lav.. One is tiptoeing along the edge of a very deep legal chasm when one makes decisions about someone else medical problems--such as, overactive bladder. Even if we are in sterile cockpit prior to landing, I will still inform the captain that there is a passenger out of his/her seat and why. I've never had one abort a landing over someone being in the lav.
An FAA cabin safety inspector said to me once, 'You can't make them return to their seats, and besides it's not your job. Just make sure you don't even give them even implied permission to be up." He then pointed out that those f/as who say "Just be careful" or "You're up at your own risk' (which they have no legal standing to say) have just given the passenger implied permission to be up. That makes the company and the flight attendant liable for any injuries the passenger may sustain.
The Inspector told me to just keep saying "the seat belt sign is on" and if they get horsey and say "I heard you the first 3 times, can I go to the rest room", the response should be 'Sir or Madam, what I can legally say to you is "The seat belt sign is on." Then if they persist and go on to the lav, any injuries they sustain are not chargeable to the company or the flight attendant.
