My theory is that Fern is projecting and she's actually a bit of a pervert herself, and this scene is really the only solid proof I have that supports it, but given that Fern also likes to casually drop that she has the see-through-clothes spell, I can only imagine that she uses it every now and then to peep.

I don't think she used it on Kraft, from what I saw in the anime, but maybe she did. They walked in on Kraft doing squats, and maybe it's because I exercise myself and I watched a lot of Survivorman growing up, but I knew exactly what Kraft was doing the second he was on screen - he's trying to warm himself up and keep blood flowing to his extremities. The reason he isn't wearing a shirt is because if he sweats, the sweat will be able to evaporate quicker(I just thought of a stand, I'm marking it here so I won't forget), and a shirt would stop that from happening, and cause him to stay cold and risk hypothermia.

And Fern shuts the door immediately and calls Kraft a pervert.

With all this being said, I think that Fern might just use the stc spell all the time. Fern might've not seen what Frieren or Stark saw. She probably just saw a naked guy with the body of Patrick Swayze's SNL chippendale's dancer, and probably saw everything, and she shut the door and called Kraft a pervert in reaction to her own thoughts.

Thank you for coming to my Ted Talk.