Ah, part 4.
Hmm, well, I guess you can say she was partly just 'making sure he's good enough' for her friend, but considering it involved her getting in on the action and having her fun too, I think it was also partly motivated by her just genuinely wanting a taste.
Page 10 - Mayu and Chigusa: "This Is Your Punishment."
Me: "THIS ... IS ..... PUNISHMENT?"
So, In The End, does he qualify as a Lucky Bastard (threesome route unlocked), an Unlucky Bastard (jealous lover engaged), or Why Not Both? You Tell Us, Sensei ... You Tell Us.