There is a whole bit later about how difficult it is for an outsider girl to tell the difference between guy friends roughhousing and one guy in a group getting one-sidedly punched when it's happening in plain sight that makes some sense even if she had seen everything.
The WN makes it a bit more clear that she only recognized him visually as a classmate and knew nothing else about him.
EDIT: I do have to add that she gets a bit of an infodump about him from her bestie between that first meeting outside the dungeon and their next where she decides to accompany him. She's impressed that someone with that as his backstory would have the courage to face the dungeon alone, but that wasn't exactly the other girl's intention. Not by a longshot.
This makes some things better but I hope they SHOW that in the manga cause in this medium she just looks like an asshole that ignored him being tormented. Also considering the bully was tormenting him directly in the classroom while she was there, theres no way she never noticed him getting physically attacked like he was.
Girl literally just pretend she didnt see him getting bullied and beat up in front of her, but then has the gall to ask to help her in the dungeon where you could die anytime, how would anyone trust her with their back 😐
The WN makes it a bit more clear that she only recognized him visually as a classmate and knew nothing else about him.
EDIT: I do have to add that she gets a bit of an infodump about him from her bestie between that first meeting outside the dungeon and their next where she decides to accompany him. She's impressed that someone with that as his backstory would have the courage to face the dungeon alone, but that wasn't exactly the other girl's intention. Not by a longshot.