Woman (20sF) at gym is flirting and making my( 27F) BF (26) uncomfortable. How do we get her to stop?

Obligatory sorry for the throwaway delay bad formatting.
My bf has been going to the gym consistently since right after lockdown started and I?m happy for him and the confidence it?s given him over the past year. He was never overweight, but he?s more toned and lean and I catch other woman looking at him all of the time. Ok whatever
We recently moved to a new city and he started going to a new gym. The first day he was there he told me that a girl we?ll call ?J? asked him to help spot her on her squats. He didn?t know what to say (what he told me) so he spotted her. He told me that she was obviously trying to flirt with him and get him to look at her ass. So afterwards he chatted with her to not make it awkward I guess and left soon thereafter. No numbers/names exchanged.
The next day he didn?t end up going to the gym because he didn?t want to run into her again. He was thinking about going to another gym but he is already a member of the chain he currently goes to and they have all of the equipment he needs. So after skipping a few days he went back yesterday, at a different time and K is there.
He tells me that K once again strikes up a conversation and this time asks to hang out. My bf tells her that he has a gf and she starts to get red in the face and look embarrassed. She then tells him that he looks like ?someone that would cheat? and he laughed it off and left the gym and hasn?t been back.
We don?t really know what to do from here. He is scared to even go back because I honestly don?t think he knows how to reject anybody and is afraid that if he does it will be awkward. Any advice?
TLDR: BF getting hit on by girl at new gym. We don?t know the best course of action to take


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