From Holmes' account, Marvin learned about the events after entering the Ghost Forest.
Because the specific location could not be ascertained, Holmes had the guides make thorough preparations; they brought tents, food, and many pieces of equipment used for mountain trekking. As for medicines, since they were accompanied by a priest who was a follower of the Goddess of Health Verthandi, there was no need to prepare too much in the way of medication.
The involvement of supernatural power in the Ghost Forest meant Holmes certainly wouldn't be foolish enough to venture in alone. Relying on his fame, he asked a priest from the local Tri-Goddess Church to accompany them. The United States of Aurous was a multi-religious country, unlike the Kingdom of Windsor and the Kingdom of Frederick, which had state religions; the churches within the country were composed of various denominations.