O2 sensor question
 

I have a friend who is running a haltech e6x and had the car tuned by a shop. The shop told him he had very weak signal from his o2 and they tuned the car with the o2 sensor unplugged. What would cause a weak signal and can a haltech be tuned without a o2 sensor?

They probably used a tail pipe sniffer WBO2 setup. It almost sounds like your saying they were going to use a narrow band sensor to tune the car which is a no no anyway. Even so a weak signal doesn't make a lot of sense. There either is a voltage or not, could be intermittent or even incorrect but there is very little current with the voltage coming from a narrow band O2.

He got his car back and the O2 sensor is unplugged. The shop told him he can put a bung in place of the sensor. Have you guy's ever seen this done?

Yes very common, I do it on most tunes as I rarely use closed loop.