do you think she a good friend?

My sister friend keeps calling & texting my sister boyfriend. Which her boyfriend & friend use to be sex buddy's & talk. My sister didn't know until 5 months later of their relationship. Just to let you know my sister & her friend was FRIENDS since intermediate now they both pretty much in high school. My sister boyfriend keep saying she a good friend. BUT , my sister feels uncomfortable cause the girl KEEPS constantly texting or calling. It bothers her cause it's like constantly. My sister told her friend she feels uncomfortable. The friend ignores her & talks to the boyfriend again. My sister cried to me cause that's her friend & her boyfriend. So I message her friend saying it is pretty much disrespectful she ignore my sister feelings. She told me "Idgaf bitch, I text him whenever & whatever I want! If I wanna fuck Him I will! " made me pisst off pretty much.i told my sister that's not a good friend. & her boyfriend is trash for letting the girl come to him like that & disrespect the relationship like that. TBH I've never seen a friend TEXT another man CONSTANTLY! Everyday & night kinda thing. I just want to know how tf this girl is a good friend? Really I want to know? All I see is a HOE friend from this girl. & oh she claims my sister boyfriend is her BESTFRIEND when my sister saved her ASS through out these years. She'll come to our house whenever her & her mom fought. I felt weird around her like a snake vibe. I told my sister she a snake 🐍 sneaky. But my sister claims she a good friend. Then one day she said my sister ain't her best friend my sister boyfriend is her best friend. They're SO fucking weird! I told her that! I just scared for my sister especially she living with her boyfriend!