The identity of the player has not been announced while the team contacts his family in South Africa, SA Rugby officials told Reuters on Friday.
French media reports said the player was found unconscious in a shower cubicle at a hotel on the Ile de Re in France, where the team had been staying since Tuesday.
Emergency services were called shortly after 10 a.m. and attended to the player, who was in cardiorespiratory arrest. Attempts to resuscitate him were unsuccessful, the reports added.
The Lions were due to make their debut in the competition, but the match is now in doubt, as are two more matches in Scotland in the United Rugby Championship that they were due to play over the next fortnight.
The death comes 10 years to the day that Munster coach Anthony Foley passed away at the age of 42 in Paris ahead of a Heineken Cup match against compatriot Ronan O’Gara’s Racing 92.
In a sad coincidence, O’Gara is now the coach of La Rochelle and according to Irish media visited the Lions at their hotel on Friday to offer his condolences.
The Lions did not respond to requests for comment, while European Rugby confirmed no decision had yet been made on the fixture against La Rochelle and will be done so in consultation with the Lions.