Here is a list of the ones I've found. There might be a few missing, so sorry about that. I'll update it if I find more, or if a comment under my post mentions anything:
- Plus
- Minus
- Aroma Veil
- Battery
- Flower Git
- Flower Veil
- Friend Guard
- Healer
- Neutralizing Gas
- Pastel Veil
- Steely Spirit
- Sweet Veil
- Symbiosis
- Victory Star
In addition all weather causing abilities like the ones below, can help out Pokémon in certain circumstances, like if they are the same type, or have an ability such as Slush Rush or Sand Rush.
- Drizzle
- Drought
- Sand Stream
- Snow Warning
- Desolate Land
- Primordial Sea
Air Lock*
Grassy Surge
- Misty Surge
- Electric Surge
- Psychic Surge
Also, if one ally has Telepathy, it can dodge attacks from its other ally, like Surf or Earthquake.
*Sometimes, eliminating weather could be helpful to your partner, as well.
Edit: In rare scenarios, like with a Slaking or other Pokémon with a bad ability, Mummy or Wandering Spirit can help the ally, although the latter may also harm the Runerigus. Unfortunely, the ally must make contact with the Cofagrigus or Runerigus to activate the ability.
Sources: Pokemon Database , Bulbapedia , Kyogre's Bulbapedia , and Cofagrigus's Bulbapedia
I hope I helped!
