advice for my bro on how to friendzone a girl he doesnt like anymore who likes him

My little bro recently asked me on advice ob how to friendzone a girl he doesnt like anymore who likes him. I mean if I'm being honest I dont knkw why he'd ask me, I'm 16 years old and never had a girlfriend or been close to one.

But yeah basically heres how he ended up in his situation:
He was with his friends at the cinemas and these girls came and approached him when he was with his friends. There was an equal amount of girls to boys and they were all in his year. 1 of the girls fancies my brother and 1 of the others fancies his friend. So, at that time my brother thought she was fit so he spoke to her a lot and didnt give her dead responses when she spoke to him. Only thing is though after talking to her my brother thinks shes a nice person and they get along but my brother isnt interested in dating her. However his mate is interested in dating the girl that likes him. Only thing is though those girls had a conversation with my bro and his mates and told them that last time someone one of those girls were talking to (as in talking inbetween being friends and dating) wanted to end things they all got mad and chased him or something like that. Then he was in a groupchat with them all and they roasted him and his family and my bro is scared of friendzoning this girl because he thinks that she will do the same with her mates to him because hes not into her. She would send my bro messages saying miss you and all that even though they werent dating which I'm a bit confused with.

So how can my bro friendzone this girl who thinks he likes her and wants to date her without being seen as a jerk and without making them angry as my bro doesnt want to upset the girl but just isnt interested in her anymore.