Other issues
His negative equity should be adjusted for the tracker
If the mortgage was on a Standard Variable Rate, the true negative equity would be €80,000. However, a tracker of €220,000 is probably the equivalent of around €200,000 on a Standard Variable Rate.
So the true negative equity is about €60,000 and his share €30,000
Can you rent a room?
If you rent a room, it will contribute to the cost of the repayments. Keep a full record of all rent received and payments made in case they are needed at a later stage.
Can you rent out the house?
You could rent out the house in its entirety if the rent received covered the mortgage repayments. This would be a lot of hassle, but worth considering.