My recollection is similar - IIRC the pax was an experienced flyer (gliders, perhaps) and was happy to strap himself in. Unfortunately he missed the crotch-strap which as they flew past (can't remember whether aeros were involved) fell & got caught in the gap between stick & floor. No amount of force would have been able to free the stick.
I would point out though that this is just what I recall. I am quite happy to be corrected if anyone knows better. The whole thing is vey sad.
ap
In fact, just found the AAIB report that HM posted before. Pax was NPPL, and
A friend of the passenger assisted him into the aircraft
and noticed that he did not secure the shoulder straps of
the seven-point harness. He offered him assistance, but
the passenger declined, stating that he would not require
the shoulder straps. It was not established whether the
pilot was aware that his passenger was wearing only
the lap strap element of the harness; it was established
during the investigation that the crotch strap was also
unsecured.
Full report
here