Hi Amazo,
If you client is registering for VAT late then they are opening themselves up to potential late registration penalties.
Please see the guidance below:
Calculating and notifying penalties
When the client registers for VAT then they are allowed to reclaim preregistration VAT as input tax subject to the conditions below:
Input tax when VAT paid on goods and services received before VAT registration
If the client registers for the Flat Rate scheme then input tax is blocked except for the purchase of large capital items.
Please see the guidance below:
Capital expenditure goods
However, joining the Flat Rate Scheme will not prevent the recovery of preregistration VAT as input tax as subject to the normal rules.