I weigh 315, would like to have a step through, but have read where test riders at half my weight are talking about "frame flex". Not exactly confidence inspiring, even when riding within the design limits. I went conventional for that reason....
Spoke size might be part of the formula, but I think I'm more concerned about the design. That's me though, FWIW.
I have the Pedego Platinum Interceptor step thru with the mag wheel option. The main reason I chose the bike was it's 350# weight rating. I weigh 260 but usually carry a lot of gear which brings the total to over 300#. Yes, there is some frame flex but not enough to affect the bike's handling. Over 3000 miles so far with no issues.
When I was bike shopping, a LBS mechanic told me the spokes are the main weight limiting factor. He recommends bikes with a 36 spoke format for heavy riders. The spoke gauge and material make up are also factors.
Generally speaking the listed weight limit is what the wheel build (spoke pattern, rim, spoke thickness, and spoke count) is designed to handle. The frames and motors can handle significantly higher weight loads. The thing an "industrial sized individual" will need to monitor is whether the wheels are going out of true or not. If they do go out of true quickly you can take them to a bike shop and have them re-laced with a spoke pattern that can handle a higher load.
Long story short, if you would like to let your 350 pound friend test ride your bike, the bike should be absolutely fine.