I don’t understand. Isn’t this repair a job for the OC? You didn’t cause the leak somehow, did you?
I’d guess you (i.e. the OC) need to gut the bathroom, treat any concrete cancer, re-top the slab, apply a waterproof membrane, then replace the fittings and re-tile.
Meanwhile, the unit is uninhabitable and they have to pay you market rent on it.
The tenant will have to stay with Mum but doesn’t have to pay rent to you.