A home built in 1955 is less likely to have asbestos. The popcorn is likely a form of plaster. Apartments with asbestos are required to have an O & M plan for managing the asbestos. Typically asbestos is not a problem unless it is damaged or needs to be removed. If you have asbestos, It would affect repairs where asbestos needs to be removed. Workmen would need a respirator while working with the asbestos, and the asbestos would need to be bagged and disposed of with a special landfill. In such case, you would hire an asbestos removal company although some contractors are equipped to work around asbestos. Since it is unlikely that the ceiling contains asbestos, and the ceiling material is not friable state (it is not flaking), you could inform the tenant that you have no reason to believe the ceiling contains asbestos and if it did, that the asbestos is friable. You do have a leak and I would be more worried about mold.