This woman is definitely overstepping her bounds. He made it clear to her that he does -not- want any other romantic relationship. This girl has already made it clear that she is not willing to respect his wishes, and seems to be willing to do whatever it takes to get into his pants, even force him to do so it seems. :/
Her blatant flattery reeks of her attempts to go against his wishes and attempt to flirt with him anyway, in some false hope that he will "cave in and give into her so she can get laid with him."
Any relationship, poly or mono, is about trust - trust between all partners involved. And this woman clearly does not respect him, at all. She just wants in his drawers and wants the D, and will do anything to get to it.
I would remove this said woman and block her from any sort of social site he has her added on. Also I would recommend for him to call block her if he has her cell. Definitely steer away from this woman! No friend would do this. While I understand he wants friends, he needs to attempt to find people that won't flirt with him and respect his wishes to remain mono.