C30 does not start

Hello professionals, my partner drives a C3 136PS automatic diesel. The car has always been in service until today, but today he simply didn’t want to start. When starting the car it only makes once crack … and then nothing more. Try again, the same result. After repeated attempts he does not make any more crack. At first I thought that the battery was empty, but one of Volvo’s mechanic had even connected two batteries (as a whole 24V) and he has been m not to start the race – he said that the starter might have to be replaced. What do you mean “by the professional world” about this? Look forward to your answers. LG Karsten Uwe