If their purchasers are intending to put the property to residential use, your friend is unlikely to need to charge VAT. The evidence required depends on the exact circumstances – for example, the buyer may need to complete a certificate. It’s worth reading sections 3.1 to 3.4 of HMRC’s notice 742A to work out what they need to do and keep: https://www.gov.uk/guidance/opting-to-tax-land-and-buildings-notice-742a#sect3
Having established that the sale is not subject to VAT, they need to determine whether it is zero rated or exempt – as this impacts on their ability to recover VAT on related expenses. If the sale is zero rated, they are entitled to reclaim VAT on any related costs – whereas if it’s exempt, they can’t. Probably their best chance of getting zero rated treatment would be if it was regarded as a “non-residential conversion” – see section 5 of notice 708: https://www.gov.uk/guidance/buildings-and-construction-vat-notice-708#section5
If they’re unable to get zero rating and the sale ends up having to be VAT exempt, they’ll need to consider whether there will be any “clawback” of VAT that they’ve reclaimed from HMRC in the past. The logic behind clawbacks is that VAT may have been recovered on expenditure on the assumption that it fully related to the “VATable” holiday lets – when in fact some of it would now relate to a VAT exempt sale. Details on the clawback are in sections 13.7 and 13.8 of notice 706: https://www.gov.uk/guidance/partial-exemption-vat-notice-706#section13. There are also separate rules called the “capital goods scheme” (notice 706/2) which may also require some adjustments to be made if any works valued at more than £250,000 have been carried out on the building in the past 10 years. But all of this only applies if the sale is VAT exempt. If it is zero rated, there is no potential risk of these clawbacks or adjustments.
Obviously a lot is dependent on the specific detail – but hopefully this gives you / them somewhere to start!