The best way to proceed will depend on your client’s overall objective. That is to say, does he want to own the property for his residential use, or to use it as an investment?
The property sounds like a single dwelling which at an earlier date has been converted into two separate flats by a previous owner, in order to maximise the property’s potential value.
In an active market, two leasehold flats will be worth more than one freehold house, even with the cost of conversion. In this situation, there will be two separate legal titles and a lease, which will deal with the usual matters relating to common parts and responsibilities.
If he wants to buy the downstairs flat as an investment and let it out, then he should take a separate buy-to-let mortgage and leave the physical arrangements as they are.
If he wants to own the whole building and use it all as his own residential home, he will need to buy the downstairs flat, probably using short-term finance.
Then he will need to make physical alterations to the property to convert it back to a single dwelling unit, and then remortgage the whole building, while recreating a single title to the property and changing the tenure back to freehold from leasehold.
Frankly, he would be mad to go through such pain, unless he has the opportunity to buy the downstairs flat at an incredibly reduced price.
He would be better advised to sell his existing flat and find a suitable house to buy.