Source: The Great Interpretation of Dreams (Ibn Sirin)
Praying for the dead in a dream indicates the reward of the prayer reaching them and their need for it.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Praying for the dead indicates the righteousness and piety of the dreamer.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Praying for the dead is interpreted as good for the deceased and the one praying.