I THINK that the catholic church teaches that once a person dies they can no longer help those on earth. It is those left on earth that can help the departed by asking God to remember their life...
The second part, yes. The departed not-yet-saints undergoing purification in purgatory can no longer help themselves, so the living should pray for them that they may be prepared to meet the Lord face-to-face (2 Mac 12 29-46, 2 Tim 1 16-18).
The first part, no. Once they arrive in the Lord's presence, i.e. become saints, they will bring prayers on behalf of the living to the Lord (Rev. 5 8, Rev. 8 3-4). We do not know who all the saints are, but we can know some who are because their intercession has been effective, demonstrating that they are in the Lord's presence and bring prayers to Him on our behalf.