I am sorry to hear that your customer is experiencing problems.
The problem is most likely being caused due to the Laptop using a switch power supply, along with no earth cable in the mains lead. There are two possible solutions:
1. Try to run the laptop of batteries, however, some laptops seem to reduce the amount of voltage levels to the serial port thus stopping the TC-08 working.
2. Use the laptop form the mains, but connect a cable from the 9D connector to a known and tested grounding point, this grounding point should be checked with an earth tester.