Genrally with fanwork, either ignore it or go "that's nice thanks" or you're heading down a slipper slope of getting too involved. On the specific issue of tracing, most of the time they'll grow out of it. Lots of artists traced when they were young/just starting out because they like the style and want to recreate it. They'll trace less and less and make more of it their own as they learn, going from tracing to copying to just lots of references. It's if they don't seem to move on you want to get concerned. But if you get involved with tracing, what about partial tracing? Copying the pose? Where does the line between what you get involved in and not as a creator come.
But that leads into creators trying to police their fandoms. Even in small fandoms, if creators try to get too involved in controlling what's being generated by their fans, putting in rules, it can quickly turn a fandom toxic. I don't even remember what book it was, but a few years back I was part of a tiny fandom for a New Adult book. The author got involved saying what they did and didn't want fans doing as fanwork (aka don't ship this, no incest, this ship is abuse, don't copy this) and within a few months the fandom had gone from "do you thing as long as it's not hurting anyone" to callout posts for people doing anything even vaguely "problematic" or "against the authors will". Obviously, it's rare for that to happen until you get at least a reasonable sized fandom, but just be careful when saying anything other than "thanks that nice" about fanwork. It's a really slippery slope. Sorry that was a slight ramble but I think these are things to be aware of as creators.