He needs to be very careful before deciding to do this.
1) Having a dedicated office within one's home lays you open to having your home being declared dual use for council tax and business rates. It means you pay a slightly smaller council tax but a whole lot of business rates.
2) The implication of 1) above is that when he sells, a proportion of any capital gain on the property is not fully exempt from CGT, because It is not all his principal private residence.
For a potential saving in tax of £480 (20% of the £2400), I don't reckon it is worth it.
Besides ALL of the sum would have had to be a repair requirement on the roof of the business office, and this sounds like the cost of the whole roof works including his residential dwelling.