You don’t have to get up every 15 minutes. If they have to go to the restroom every 15 minutes, then that is their problem, not yours. If they do not want to cooperate, that is also their problem.
FYI, you are obliged to follow crew instructions, not passenger instructions.
The crux of the question is when can a customer, in economy class, ask to be upgraded. In which situations?
How could {I / my spouse / we / any kin} have convinced them to let {me/her/him/us} sit in another class?
First and foremost, it seems that the OP should tell their family members that being upgraded to a seat due to an inconvenience is the exception, not the rule. If it were so easy to be upgraded then passengers would ask to be moved whenever they are seated near or next to…
In all the above scenarios, the only thing a hapless traveller can hope for is to ask the offender to be more considerate (excluding the obese seatmate or the ill passenger) and/or to talk to one of the flight attendants on board and hope they can find a solution. And sometimes there is no solution, there is no remedy. You just have to suck it up.
When can you get an upgrade? Is answered here:
What techniques, tricks or otherwise have you used to get upgrades on flights?
And can you sneak into first/business class is answered here:
What are the chances of success and the consequences of sneaking into business class?
As @gsamaras says, there is nothing the cabin crew can or should do. However I do think your grandma was a little passive. Her asking “Would you like to change seats?” indicates that it’s no bother to her, she’s just asking for the other person’s convenience.
What would be worth trying, after the exchange in the question is for her to say:
Actually it’s very inconvenient for me if I have to get up and let you out every fifteen minutes. I’m hoping to sleep. I think it would be much better for both of us if we changed seats.
What ought the cabin crew do?
Nothing.
I think that your best bet, if you ever got yourself in a situation like this again, would be to ask the crew, if you could be re-seated, if the flight is not fully booked. Of course, in the same class as the one of your purchased ticket.
Next time, please book a window seat (you can pay a bit extra for early check in, in order to make sure that you get it, but in general, if you check in as fast as the free check in opens, then you are going to find a window seat).
I assume that passenger was sitting in his seat (and not at an empty seat, where you argue to the crew that he is not sitting in his assigned seat), and has the right to use the bathroom at will, when the seatbelt sign is off.
My grandma doesn’t like window seat
Then, as I said, your best bet is that someone from the crew reallocates her, after she informed them. However, notice that they don’t have to do anything, but the grandma can target the human soul, and get a favor.
Credit:stackoverflow.com‘