A grieving mother in France is to sue a couple who allegedly mocked and filmed her drunken son who was later found drowned.

Sylvie Zecca, a former police officer, says she wants to make an example of the pair, and accuses them of contravening a French law requiring individuals to help anyone in danger.

Her 19-year-old son, Vincent, disappeared after a night out in Bordeaux in March 2012. Three weeks later police divers retrieved his body from the Garonne river that runs through the city.

At first the family thought he had been murdered, as one of his credit cards appeared to have been stolen and used that evening. A police investigation concluded he had accidentally drowned after slipping into the Garonne while drunk.

After gaining access to the police file shortly before Christmas, Zecca announced she was suing the two passersby who told police they had seen her son.

The pair, who have not been named, told investigators the teenager was "very drunk … near comatose". Instead of helping him, the couple apparently laughed at him, filmed him with their smartphone and watched him walk off.

The lawsuit for "deliberately not helping someone in danger" and "voluntary manslaughter" is being lodged against "persons unknown" to allow a wider investigation, but Zecca is clear on whom she is targeting. "I'm not seeking punishment, just that they be made to face up to their responsibilities," Zecca told French journalists.

The police file showed that several people had come across Vincent Zecca, but only the couple stopped to film him. After watching the short video, Zecca said her son was "not himself". She said: "Vincent was being inoffensive. He was only asking to return home."

Zecca claimed young drunks should be seen as vulnerable people as opposed to culprits. "Someone who's not walking straight in the street, is shouting, falling over, is speaking incoherently, being sick … is that funny? People say 'oh, he's had a skinful' and don't do anything to help. But when someone has an epileptic fit, you call the emergency services. When someone is bleeding from an injury after a fight, you don't laugh, you call for help," Zecca told Sud Ouest newspaper.

Zecca has set up an association, Jeunesse Volée (Stolen Youth), to raise awareness of the risks of alcohol and drugs for youngsters.

Under article 223-6 of the French criminal code, "non-assistance à personne en danger" carries a civil and criminal liability. In the criminal courts the penalty is up to five years in jail and a fine of up to €75,000 (£62,000). In the civil courts judges can order compensation to the victim or their family.

Photographers at the scene of the car crash in Paris in 1997 in which Diana, Princess of Wales, died were investigated for the offence but not charged.