Where does the line get drawn?
It is drawn by the underwriters of the drivers insurance company. If they can figure a way not to pay they will take it. If there is even a hint of a possibility that the event is commercial and the driver hasn't invested in commercial insurance then your friend is SOL if injured. Say the right words or wrong words in a police report and it's done.
A few years ago I was hit by a 2x4 that flew off of a van. The driver of the van mentioned to the officer that he had just left a house he was painting and forgot to remove the board from the top of the van. His insurance company called me a few days later to tell me that his personal auto insurance wasn't going to cover me since he was obviously using the van for work and didn't have commercial insurance. I slip of his tongue, and I was screwed. Yes my uninsured coverage kicked in this case but that wouldn't be an Uber option.
I guess your one friend could sue your the other friend to cover the health deductible if needed but that'd be uncomfortable for all 3 of you.