Fun idea! I'll leave someone else with the responsibility of tackling Sage and choose the easy option, Bianca.
What a fucking terrible best friend
Jill gives her the support she needs when she is at her lowest ebb, she is devestated and notes how thankful she is for her support during such a difficult time. They even go on a skiing trip just to get away from it all.
And how does she reward Jill for her love and support?
Trying to fuck Jill's boyfriend so that she goes through the same devestating experience
Bianca would willingly put the person that supported her most through her partner's infidelity through same horrible experience that she just went through...