She was entitled to a refund or vouchers. She opted for vouchers, possibly due to BA doing everything it could to not refund (long timelines, incentives for vouchers, dark patterns).
It seems perfectly reasonable to expect to be able to change your mind on this within the relevant time period, regardless of circumstances.
In the US, airline-canceled or "significantly delayed" flights (as opposed to passenger-canceled) are entitled to a full refund. The airline will not tend to tell you this, and they'll resist quite a bit, but you're legally entitled to one, even on a non-refundable ticket.
Under EU law (this was before Brexit) you are entitled to a full refund if the flight is cancelled by the airline. If the flight is cancelled less than two weeks before departure you are also entitled to compensation of a few hundred Euro.
I actually got a cash refund from Ryanair, although it took a long time and at every stage they recommended I take vouchers that would have expired after 12 months instead.
It's still British law for now. I've not heard that it's planned to be cut in the mass-cut of EU laws, but given that it's pro-consumer it may well be.
Keep in mind UK law. The airline cancelled so the customer can insist on cash refund.
I spent a week or two messaging with BA via Twitter DMs to ensure I got a cash refund, rather than dealing with their website which was steering everyone to vouchers.
It seems perfectly reasonable to expect to be able to change your mind on this within the relevant time period, regardless of circumstances.