Well @HistoPol, I've been trying to nudge my friend for several days into sharing some information with you, but he has unexpectedly been more reluctant than I anticipated and seems very unlikely to be creating a Mastodon account to facilitate it.
The best I could get out of him was specific mention of Peter Vermeulen and one or more of his books. The originals may not be in English, so you might have to wait for translations. Searching for those might be a next step.