Marius is inspected by a doctor and is pronounced dead right when Gillenormand, his grandfather, hear his say it and starts going crazy. While this happens Javert tells JeanValjean he's taking him and Jean asks him for a favour
im taking you JeanValjean
just let me see Cosette
Examination says Marius is dead
Gillenormand starts getting angry that Marius was fighting with the rebellion and tells him we won't grieve for his death, as he says that Marius opens his eyes. By this point, Javert and JeanValjean left to look for Cosette
He opened his eyes! He's alive!
I won't grieve over his death
JeanValjean tells Cosette he has to leave and probably won't ever see her again but he can't bear to tell her why. Javert is waiting thoughtful for JeanValjean