Glad your neighbor didn't get hurt too badly and thanks for posting this.
Who knows what really went wrong, other than too much powder and not enough checking.
IMHO, reading data directly from the manual could have been the cause, but I can't say for sure. The reason I say that is because I think loading out of the book is risky (probably a better way to say that). When we look up data in the manual, we're looking at a chart for a specific cartridge, projectile weight, but many powders and sometimes (depending on manual) different OAL's. When scanning down the chart to your powder choice, then scanning over to the proper charge weight, all while setting up equipment adds risk IMHO. When I work up a new load, all data from the manual is written in a notebook and triple+ checked and all I'm doing at that time is focusing on data, not equipment. One load workup per page and one notebook per cart! I then list additional notes and my goals for this load and below that is the loadings with space left to document performance. After I come up with a load, I put the data for that load on an index card; the cards are stored in a index card box and the one in use is hung above the press. I treat the load cards with the same respect that I do powders, projectiles...., only the one that I'm using is out, the rest are stored away.
I'll go ahead and piss some folks off while I'm at it
(don't want to, just figure it will). Digital Scales: They serve a purpose, but they are more risky to use than a good balance beam. For me, when I'm loading, I'm more focused on the smaller details; maybe I'm wired wrong, I don't know??? When I place a pan with powder on a digital scale, I pay attention to the number on the left of the decimal point, but I can't help but be more focused on the 1/10 grain. I bet this is true for most of us, admit it or not. When I place that pan of powder on a balance beam, the first thing I notice is whether it's close, while I wait for it to balance out to see if it's dead on.