Brendan,
theo could be right - this paragraph follows on from the one I quoted in the means assessment guideline: The underlined bit (my underline) may apply in this case.
If a portion of the house is being let or is being used for business purposes the following should apply. Deductions from profits should be allowed in respect of ground rent and mortgage interest (not the full amount of the loan repayment) in proportion to the area used for business purposes. If rooms are let furnished, 5% of the gross amount received is allowed for wear and tear and 15% of the gross receipts maybe allowed for voids (i.e. periods when the accommodation is vacant between lettings).
I misread the paragraph as applying only to letting for business purposes - apologies (and thanks to theo).
The wear and tear and void allowance may already have been applied in the assessment of your friend's Jobseeker's Allowance payment.