No - interest isn't a capital allowance, it is an expense.
Yes - interest is deductible for USC, in the same way as it is for Income Tax.
Tell your accountant friend to enter the figures onto a Form 11 on ROS offline or whatever tax software he uses, and he'll see the USC calculated on the net rental income.
Section 97 of the Tax Acts provides the computational rules for calculating income from rents - it doesn't distinguish interest from other expenses, except that only 75% is allowed (subject to PRTB registration). The USC provisions linked in the other thread would have to explicitly require interest be added back onto rental income, and they don't, therefore rental income liable to USC is the same amount liable to income tax (which is the amount after deductible interest).