Now he has to be careful because he has no intention of telling her that Samantha is the other woman. That's meant to be a secret and he's certain that Lydia will react badly to it.

"You've probably guessed that I have done a little snooping on our new President. I reported everything I discovered to the Mother of Witches and it was her decree that I approach those affected. I have not revealed your secret to anyone else. Nor can I tell you the identity of the President's other lover. It's all rather sordid if you ask me, but I suppose this is her way of protecting you. No one else has to know."

Which should explain why he's the one talking to her instead of some other witch. Of course Samantha could have talked to her directly, but given the terms Lydia has used to describe her today Mogget suspects he knows why she hasn't. There seems to be some bad blood between them.