Normally, to get the VAT back you have to make a taxable supply of the property which is a lease of 10 years or more (generally leases made for 20+ years so no reversionary VAT for the landlord)
Or,
You can short let the property (lease of less than 10 years) but this is normally VAT exempt so the landlord has to waive their exemption from VAT for short term lettings. Care needed because wiaver applies to ALL properties short let by landlord - apart from obscure case re Holiday Homes, so if other short let properties then need to start accounting for the VAT on the rental income - cure is to hold other properties 99%/1% with the wife.
In the specific facts of the case, you as joint buyer and sole occupant would be self-supplying your half of the property so no VAT back for you. Best your friend could do would be to get VAT back on his half, provided he waives the exemption. However, he would want to watch what would happen if ye agree to sell later, he doesnt want his half to be caught in the VAT net such that he would have to account for VAT on his half of the sale proceeds.
I'd say its hardly worth it unless your firend is really stuck for funds. Its a very complex area and you would need specific advice, the cost of which isnt cheap, plus you may have to unwind it later on before a sale, more advice which aint cheap. Best off out of it I'd say.