I would be skeptical of what the AA man told you. Amp-clamp type of meters measure the magnetic field around the wire, not the current through the wire. Because of this they are inaccurate at current levels below about 1 amp. The earths magnetic field can throw off the measurement by 250 to 500 ma. The better ones can be zero'd to compensate for the earths magnetic field but that adjustment only holds until you move the meter. Modern cars have a lot of electronics that draw a very low level of current all the time. When power is first applied to them, they draw a much higher current for a period of time before it drops to the keep-alive level. Could this have been what he was reading?
You said that you needed roadside assistance to start the car but you didn't say anything that could help decide how to trouble shoot your problem. Were you driving when the battery went dead? Had the car been sitting for a while and if so, how long? How old is the battery?
In any case, don't buy a battery from the AA man. Take your car to a good mechanic, tell him what happened and let him find the trouble.
A point of information: I once had a mechanic tell me I had a bad battery after trying to 'load test' a dead battery. He didn't charge the battery first and you can't load test a battery that can't supply current. The problem wasn't the battery, it was the alternator.
Jack j