A person can indulge in something. Is he therefore an indulger of something or an indulger in something? Are both okay?
If both are okay, is there any difference between these two phrases or are their meanings exactly the same?
I’ve never seen either of those used. Logically, it would be in, but I’ve never seen that form. The Corpus of Contemporary American English is of the same opinion, and further points out that indulger is not widely used at all (and mostly by itself: “he was an indulger”).
The Corpus of Contemporary American English contains the following sentence:
Was Jackson an indulger in and practitioner of nostalgia?
The sentence is used only in academic context, though, with a frequency of 0.01 per million.
Looking for indulger for doesn't return any result.
Indulge in means you yourself indulge; indulge of means you allow or enable indulgence in others.
I indulge in chocolate. I am indulgent of my daughters' appetite for clothing.
I don't think "indulger" is a real word.