When I witness a theft, for example, a pickpocket in a busy RER, am I bound by law to report the theft? Am I required/protected by law to defend the victim/intervene in the act?
Anyone who, being able to prevent by immediate action a felony or a misdemeanour against the bodily integrity of a person, without risk to himself or to third parties, wilfully abstains from doing so, is punished by five years' imprisonment and a fine of €75,000. The same penalties apply to anyone who wilfully fails to offer assistance to a person in danger which he could himself provide without risk to himself or to third parties, or by initiating rescue operations.
Pocket-picking is a special case of theft that violates "bodily integrity"; so this article does seem to create a conditional legal duty to intervene. There does not appear to be a specific law creating a duty to report crimes to the police, but that might be implied by the duty to initiate rescue operations.