Hi my name is ***** ***** I have been in the plumbing field for 20 years. I will be glad to help you today. Was the pipe that is leaking left underground and uncapped? It sounds like the stop valve was likely above ground and the pipe continued underground then left uncapped? Did they replace the entire pipe or only repair a portion of it? It seems to me that if the pipe were replaced that the old pipe would no longer be connected. Are some of my assumptions here correct?
The pipe underground must be completely disconnected. It would be fine to leave the old stop valve but the line after that must be capped. Leaving a deteriorating pipe connected to an existing system is poor judgment on the plumber and should be expected to repair this issue with no additional cost. Leaving an open pipe underground is absolutely wrong. Open pipes would be for water access and must be above ground. Laws and regulations will be slightly different from region to region but they all will state, in no uncertain terms that a stop valve is not a permanent cap, a deleted pipe shall not be left connected and an open pipe must be accessible for water access.