Do they own the properties within a company wrapper, or as a private individual?
Either way there is no getting away from tax on gains except on your own primary residence. Whether or not the properties are registered with the Land Registry is irrelevant (although that'll presumably need sorting out as part of a sale).
(Vast swathes of the country are still unregistered because registration on sale was not compulsory until 1990 and on inheritance until 1998; more on this at
https://whoownsengland.o...ands-unregistered-land/ )
I doubt CGT is due on the gain from 100 years ago. More likely it'll be due on the gain since you friend presumably inherited the properties (or since March 1982 if in a company; less sure about the privately owned situation and whether 1982 is a similar cut-off but the RICS page below and pages like
https://www.fennwright.c...ues-why-do-they-matter/ suggest the same applies).
Funnily enough I've been helping a relative in a similar position. Their properties are in a company so the treatment will be different to directly held ones (gains subject to corporation tax rather than CGT), but info we've found useful on historic valuations for tax purposes:
General advice:
https://www.ricsfirms.co...nd-commercial-property/
https://www.gov.uk/gover...price-indices-in-the-uk
In practice these seemed to be the only tools which could give a useful 1982 value:
https://www.nationwide.c.../house-price-calculator
(or
https://www.nationwide.c...ice-index/download-data may have more detailed data)
https://landregistry.data.gov.uk/app/ukhpi (big green button on page takes you to the data)
Council tax banding may be useful for sanity checking valuations especially if you need a 1992 value:
https://www.moneysavinge...uncil-tax-bands-change/
For company held property, you get to apply indexation up to 2017 (but for individuals, that's not an option; maybe there are other reliefs):
https://www.gov.uk/tax-w...k-out-a-chargeable-gain
https://www.gov.uk/gover...exation-allowance-rates
https://www.gov.uk/gover...allowance-december-2017
You can get HMRC to pre-approve valuations before submitting them in a tax return:
https://www.gov.uk/gover...-for-capital-gains-cg34 And finally:
http://www.britishnewspaperarchive.co.uk/ might be useful for looking up old estate agent listings in local papers. (Much content paywalled but you should be able to get free access by a local library's PCs, or at least you could if the librarys were open).