He who has not been a stubborn accuser in prosperity should hold his peace in the face of ruin.
Hugo is pointing out how easy it is to judge people only after they fall. The irony is that many who condemn a ruined person were silent when that person was powerful. In context, this thought sits near Hugo’s larger critique of hypocrisy in politics and public life. The line draws a boundary around moral authority, saying you do not get to be cruel only when it is safe. It can resonate with readers who have watched public opinion turn fast, like a crowd looking for a target. The sentence also challenges us personally: are we brave enough to speak about wrongdoing when it costs something. The quote is an argument for consistency.