Hi
From the citizens info website.
"The total (gross) rent you get from your tenant (or tenants), which includes sums the tenant pays for food, laundry or similar goods and services, cannot exceed €10,000. If you get rental income over and above this amount, you are not entitled to the relief."
If rent is €15k you cannot claim this relief and must include the figure, less expenses, in your tax return.
Also:
"Where more than one person is entitled to benefit from the rent (for example, you and your spouse), the limit of €10,000 is divided between the individuals concerned."
This means you can only claim rent relief on income of €5,000 because you benefit from half the rent. Your friend cannot claim rent a room relief at all because he does not occupy the house.