Can he deduct a portion of the rent paid to reside at this property, for his business taxes?
Yes.
Is he only allowed to do the home office deduct, or is there more he can deduct due to using the unattached garage for business use?
A portion of utilities can be deducted too, provided they are actually used for the business. For example, a portion of the electric bill can be claimed as a business expense, if the detached garage is wired and electricity is actually used in the detached garage for business purposes. If the garage does not have water, telephone, cable, or internet, then none of those utilities are deductible as a business expense.
Understand that the property should not be listed as an asset, since neither the business owner or the business own the property. You can not depreciate that which you or the business does not actually own.
If you do elect to list the detached garage as an asset, then since you don't own the garage you'd have to enter your cost basis as ZERO so as not to depreciate that which you do not own.
You'd probably be better off treating it as rented property since that's what it really is, and claiming the portion of the rent that applies to that floor space as a business rental expense.
