@steve babiakOkay a couple of things about cast Iron pipes learned today from a master plumberDO not let them "dry out" from long term LACK of use - they exfoliate and then crackanother Do not let them "Dry out" if attached to a sewer system (instead of a septic), Some nasty sewer gas gets trapped insides the pipes and cause them to crack at the top of the circle. 

I think transparency and communication will help greatly; with the disgruntled Tenant to give them peace of mind and that you have their back and with the long term tenant, that they know you are hearing bad things about them.With that said, you do not want to exfoliate a hostile environment so if the disgruntled Tenant still wants to leave, I suppose it would come down to the verbiage in the lease and how comfortable you feel getting the place rented as quickly as possible.