It's pretty obvious from playing the Prologue that the friend was in love with the MC. There's no indication that she was slutty at this time, either. All we know is that she's currently slutty, pushing for the MC and FMC to open their relationship, knows about the MC's sharing kink, and is in love with the MC. She's obviously trying to hook up with the MC when the honeymoon is over and opening up their marriage is the way for her slide into their relationship, and she's acting slutty because she knows the MC is into that. And if the MC wasn't willing to change it from fantasy to reality, well she's not currently with the MC, so she can give him that kink without breaking any promises/vows. It's a win-win-win all around for the friend.