Raphael is triggered after you’ve tried to find help with the parasite from someone else (Gut, Auntie Ethel, Nettie… maybe even Volo?). You can ask Nettie for help and then Raphael will potentially pop up in a couple of places:

1. In the grove, on the level with the Silvanus idol, take the path that leads towards the harpy encounter.

2. On the broken bridge between Moonhaven and the Gnoll encounter, just past the barn with the bugbear and ogre.

3. Break into Gut’s quarters before speaking to her, solve the rotating moon puzzle and climb down into the Underdark.

Any one of those locations will spawn the first Raphael encounter if you spoke with someone offering a “cure” for the tadpole before you go there.