The A/C system was found with very low refrigerant, the evaporator coil is where the leak is coming from. It is very rusted and the is still leaking. It maybe time to replace. Another option would be to replace the complete system, this would save them money in the long run. Or lastly, it is possible to re-charge the system with leak sealant, and not be able to warranty it, but it is not going to last long.