Ask an Immigration Solicitor. Get an Answer ASAP.
What is their nationality?
They may apply for ILR under the 20 years long residence rule once they have been here for 20 years.
See here for more information: https://blackstonelaw.co.uk/uk-visas-and-immigration-services/family-and-private-life/20-years-long-residence.html
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You may apply, applying is one thing, but being successful is another.
My view is that they have a better chance to apply under the long residence rule.
All the best. Please leave a positive rating for my honest answer.