Received a call from Tom Tucker approximately 1600 wanting to switch the flow from TW to El Paso for testing purposes.  Tom explained that it was warranty work and the flow would only be switched for about 5-15 minutes. I agreed to allow him to switch the flow.  He agreed when he was finished testing that everything would be put back to the original set points and he would call GC when finished with the test.  Tom called GC shortly after the flow was put back to TW feed to Griffith.  He explained that it did not take long and everything was finished to his satisfaction.  Shortly afterwards at approximately 1725, I noticed that the TW feed to Griffith was falling in rate and El Paso went to 165 flow rate.  I resent the set point for TW flow to Griffith.  I called the Griffith plant and spoke to Dave (operator at the plant), who was unaware that Tom was in the yard, to check on the plant pressures and flow.  Dave had not seen any problems with pressures or flow.  I also called Tom's cell phone - reached him the second time.  He was still at the site and told me that he had by-passed our meter to Griffith.  He also told me that the reading from El Paso was false and apologized for not calling to let me know that he was performing additional work.  I asked him to call once again before leaving the site.