How can an eight month old baby break his femur. it was an accident (friend) but web searches show this is fairly rare.?

Child Abuse: Any child with a long bone fracture should prompt you to call child protective services.

Indeed rare: It is a fair amount of force to cause this. I'm thinking abuse first. Another possibility is pathologic such as osteogenesis imperfecta. Please get formal evaluation now. Possibly other injuries exist.

Bothersome: I would never accept the "accident" story unless the kid was unrestrained in a car crash.Even then, it would be a parenting failure. In my hands this would be followed by a full skeletal survey (x-rays of every bone) and an evaluation by the child and family service experts.