Leos seek the best and may try to steal their friends' partners if they're lonely.
If you see a Leo bestie flirting with your fiancé, they may make you feel unworthy of them. They may also suggest you were lucky to date them first.
Aries aren't jealous. They may want to help their pals avoid a disastrous outcome if they fall in love too quickly.
So, Aries would flirt to entice the friend's partner to test their fidelity. They are faithful pals, thus they would report back to their friends whatever.
Cancer is a romantic. They'll move in with their friend's partner without hesitation.
They would make their friend feel bad and deny breaching their confidence. Your Cancer friend would then say they never meant to hurt you.
Scorpios might be glad for their pals if they have already succeeded.
If your Scorpio BFF is single and you're talking about becoming serious with the person you've been dating, they may be jealous.