Platonic relationships and “best friends” that move towards seduction and lust are no longer friendships. They are relationships. If your partner was transparent about his changing feelings toward her, and her’s toward him in relation to you, it turns into a polyamorous situation if you agree to still trust him and his feelings for to be involved with her and you.
If he lies and she lies, and they try to hide their feelings about how they want to have more than a friendship, is it because you told him you wouldn’t stay?
Why be afraid to tell you the change of heart? There are millions of polyamorous people out there. I think. Well maybe. I dunno. I met a million at burning man.
Couldn’t you just sit him down and set the boundaries for what what was transitioning in your lives? Not hour job I suppose but you were aware of what was happening.
Don’t hide. Go seek truth.