Originally Posted by supcom

Damage to the derailleur, or other parts, caused by the other guys use of the damaged equipment is his own responsibility. He should have inspected his bike - or had a competent mechanic do so - before attempting to operate the bicycle. You did not cause the damage to the derailleur, he did through the operation of equipment that even a simple inspection would have determined to be unfit for continued use.

However, you could also take the position of, having reviewed your recollection of the circumstances surrounding the incident, you realize that the other guy was riding too closely considering that you and he had never ridden together and were not familiar with the riding style and abilities of the other and furthermore, the other guy was following too closely for the road conditions and did not allow sufficient room to avoid an emergency stop. Furthermore, when you did attempt to stop for an obstacle, the other guy recklessly passed too closely to the side of you and put himself and his bike in harms way. As such, you feel that you are not liable for damages incurred in the accident he could easily have prevented.