Mate, your original question didn't mention a 4-600 lbs (!) person. The way you framed it, including the comment on the back seat and preferring only one passenger made it appear your car was a little precious.
I'm glad it's not, and I'm glad you did a good deed. I regularly have kids, dog, bags etc in my car and I don't fret. Saying that, would I let someone who weighed nearly 17% of the weight of the car in it - no way! Unless of course it was a good friend, a dying wish or an absolute emergency. I think it would be almost a hazard, all that weight on one side. Maybe a quick spin around the block, but not a long trip.