Resident or Non-Resident for ISA ?

I know I am non-resident for tax purposes, as I have note from HMRC telling me so after three years of pointlessly filling out Annual Tax forms. I spend less than 30 days a year in the Uk and live elsewhere.

My wife has not worked for a couple of decades. She has no job and has not had one for a couple of decades however we have always kept up with her NI payments so that she keeps her entitlement. So as far as HMRC is concerned, she is at home doing nothing. She is on the electoral register at her UK home location and pays full Council taxes etc.

She does however spend a lot of time overseas.

The description of the test for who can pay into an ISA is stated as 'You must tell your ISA provider as soon as you stop being a UK resident'. My wife remains a UK resident even though she spends a lot of her time (the majority) but she has a second place where she resides for part of the time.

What is unclear to me is whether my wife can continue to invest in an ISA as she has not been declared 'non-resident for tax purposes'. Obviously she is a resident of two locations - one of which is the UK and one which is not. It is clear to me that the test for me being non-resident includes a limit of 90 days in the country such that if I go over that I lose my tax exempt status.

But as my wife in a non-taxpayer in the first instance, earns nothing but lives in two places, can she invest in an ISA ? It is unclear to me what the definition of 'as soon as you stop being a UK resident' as its surely possible to be a resident of two countries ?

The test to change your status from non-resident for tax purpose to resident is clear - but the test for the opposite is not clear.

How would HMRC know she is resident in two countries ?