Because he's dead

Well.. whatever it was, it was a GUTSY choice.

I doubt he would have told them if they had asked.
He wanted to take the credit for banishing the ghosts himself (and likely try to get a book or tv show deal out of it). He seemed to be quite pleased with his bragging rights over his accomplishments.
Maybe now that he's dead, he can teach them how to do the spell. The ghosts don't seem to have much trouble appearing to the living in this story, especially during this week of the special phase of the moon.
No Quarter

I think it wouldn't have mattered if he did; unless they had sage, parsnip and whatever else at the house there was really no need to get into it at that time.

Even still though you'd think after seeing Elias being shot with arrows right in front of them they wouldn't be taking any chances, especially with some crucial information about a spell that could stop colonists for good.

Because they didn't know he was going to be killed. Besides, they know nothing about supernatural spells to perform them.