The Texa is the dogs bollocks diagnostic unit, if your friend has the correct version and cable. Has he got a CO or AFR meter? If you set these at tickover and 3000 rpm the rest should be close enough. Obviously a tune on a rolling road is better if you can afford it. The stalling is related to mixture and stepper motor so check the stepper is free and working, then set your CO but generally if you close the throttle more slowly just above tickover it can help as the stepper can be slow to react. The engine is quite reliable as long as you check for the common faults.