Downdraft
Warning! Speculation ahead:
This plane may have had issues with weight and density altitude (and mixture management, etc). But there may be another issue as well.
In the interview the passenger stated that over the trees they encountered a downdraft. I do not doubt that they did. In fact, perhaps it should have been expected.
In my experience flying in the west, it is normal to have an updraft on the upwind side of a ridge, and a corresponding downdraft on the downwind side of the ridge. Thus, assuming they were taking off into a prevailing wind, and still flying into the wind as the approached the rising terrain, a downdraft would not be unusual. Perhaps it is another factor that may be considered.
I encountered very significant up and down drafts crossing ridges on my first two solo cross-country flights as a student. I was not prepared for them and they surprised me. Now with a little more experience, I expect them, prepare for them, and have not had any problems.
It may also be the case that the change in vegetation may have had an effect on the rising or falling air currents. Depending on the time of day, the open fields may have more heated air rising off of them than off of the tree-covered terrain. This might possibly have been perceived as an effective downdraft over the trees.
Marginal performance isn't good enough in situations like this.