What type of storm cloud is this?
I had a rather difficult time telling how the rest of the cloud structure looks; however, I would tend to believe your weather-enthusiast friend might be correct.

A wall cloud - rotating or not - can also be known as a pedestal cloud and is made up of cumulonimbus clouds; generally forming in the strongest part of the storm.
From what I can tell in the picture and judging by the weather that came from it; it sounds like it was a non-rotating wall cloud. Here is an additional reference page that may help you identify it.