Pippa has a request regarding Blissey, which once helped her out of a difficult situation.
The Request
After you catch Blissey or increase the research level of Blissey's Pokédex entry, check the blackboard in Professor Laventon's office in Galaxy Hall in Jubilife Village to accept this request.
Talk to Pippa
Go to the Alabaster Icelands and talk to Pippa at the Snowfields Camp.
Meet Up with Pippa
Go to the guidance marker and talk to Pippa there.
Battle Abomasnow
This Abomasnow can't be captured, so just defeat it.
Reward
You get 1 Shiny Stone and 2 Max Revive.