I have a cannon/hotpoint cooker though it's a ch60dpxfs bought three years ago.
I just tried turning the main oven on, dialing it down to 140 and turning off at the switch. It came back on at 140. If you use the dial to turn it on and off then it will always come on at 200 (or 190 when it's not in fan assisted mode for some reason).
A couple of other thoughts though. I used to live with a blind guy who had sticky on things for oven dials and so forth. The top oven on my cooker is a more simple turn it around to the desired temp. If you stuck one of those knobs on the dial and another just above the dial you could make it harder for someone to turn it past 140
a final point on my cooker, this could (and I hope) just be mine but I seem to have to cycle through setting the clock before getting to the timer. If that one works the same way that could be very confusing for her. (just re-read and she uses a separate timer so maybe less of a concern).
I just tried turning the main oven on, dialing it down to 140 and turning off at the switch. It came back on at 140. If you use the dial to turn it on and off then it will always come on at 200 (or 190 when it's not in fan assisted mode for some reason).
A couple of other thoughts though. I used to live with a blind guy who had sticky on things for oven dials and so forth. The top oven on my cooker is a more simple turn it around to the desired temp. If you stuck one of those knobs on the dial and another just above the dial you could make it harder for someone to turn it past 140
a final point on my cooker, this could (and I hope) just be mine but I seem to have to cycle through setting the clock before getting to the timer. If that one works the same way that could be very confusing for her. (just re-read and she uses a separate timer so maybe less of a concern).