She was registered as a non domicile, which is a perfectly legal registration process. Nondom status allows you to choose taxation basis - you can choose to be taxed in the UK on worldwide income and claim double taxation relief in the other country where you earned the income. Or you can choose remittance basis ie pay taxes in the other country where the income is earned and pay UK tax on any money she brings into the UK to the extent that the UK tax rate is higher than the other country.
In her case, she earns her income primarily as dividend in India fromher shareholding in her father's company and is required to file returns and pay taxes in India at Indian rates. Let's say, dividend is taxed at say 10% in India and 20% in the UK to keep it simple. If she earns 10million in dividend, She pays 10% tax in India. And if she brings in 1million into the UK, she paid the extra 10% (20% uk rate minus 10% India rate) to HMRC. This is the remittance basis she was doing earlier- perfectly legal.
Now she has volunteered to switch to the method where she declares worldwide income in UK and gets double taxation relief for taxes paid in India.
So, in effect, what she has done is 100% legal. Tax avoidance ie using legal means to reduce taxation is perfectly valid. Tax evasion ie hiding income or conflating expenses to reduce taxes is illegal.
Why would she pay any more tax than she absolutely had to? Which millionaire does? Its only because of the optics that she's changed her nondom tax method.
The rules are written by the MPs for their cronies. People should be critical of such loopholes, not that people are using the loopholes because they can.