OP, a few experienced SEN parents here have suggested solutions - I'm sure you'd try those first and then if they don't work, of course just politely ask the neighbour, and don't try to trip her up or manipulate her. She will likely say yes.
But I'm a bit aghast at the people calling others cunts and expecting them to set alarms on their phones. Can you honestly not see this wouldn't be possible for many?
Off the top of my head -
We have strong willed young twins who until very recently would have been bathing and going to bed together at just this time. Leaving them alone has in the past resulted in one having their head pushed under the water in the bath or a big overtired fight. We can't always just nip down.
My sister is an emergency services worker who works crazy changing shifts. She could not set a permanent alarm on her phone for 8pm as sometimes it'd be going off in her locker at work or wake her up when she was asleep. So she'd have to factor it in every day and being human and very tired, would likely forget.
Our NDN is 88 and has diabetes and cancer. She doesn't sleep well at night and increasingly sits in front of the tv around 5/6 to eat dinner and then falls asleep with tv on/ curtains open. Once, I actually wondered if she had died in her chair and went round, and she explained that as she had got older and sicker, that was her routine now and when she got her rest. I believe many older people are similar. I don't think she should have to be woken by an alarm every evening.
None of those scenarios are horrible arseholes who don't care about SEN children. They're just ordinary people.