Finally an update on this!
I thought the door was to the dungeon, but guess not, and he had to stealth his way in. His friends sure are luckier than the other explorers getting destroyed by higher level monsters. Considering that the monsters have been killing indiscriminately, I'm not sure why we should expect the girl to still be alive.
I wonder how the arachne will react to him. He hardly looks human, so I wonder if it'll recognise him as a foe, but also monsters appear to be able to detect his strength and prefer to avoid him (or at least, the red slimes could).