We have a gelding that is always rock-solid in the trailer, except when standing in a parked trailer alone. We learned this when he decided he'd been in our straight-load 2-horse for too long and he panicked. He fell onto his side, landing with all 4 legs underneath the divider. The divider got stuck so couldn't remove it. Ended up having to cut the back divider post from roof to floor. It was unforeseen and miserable. When I read about your mare going down, sitting, etc., I thought of how easily she could end up in a similar situation.
I know it's not always realistic to just "go buy a different trailer." Just sharing. Best wishes on getting your friend past her troubles.