It's not that he can't heal him, but he don't,considering he can save the dark elf with elixir (which is he must carry it with him another elixir considering how cautious he is) , that's what Diablo final monologue about, and contemplating "is this the right decision", he's respecting sasara father wish because if he save him it will denied sasara father wish, decision and his path on life. So he can only " Watch it till the end".