Hello David
As you know BnB doesn't allow hosts and guests to share contact information before a booking is confirmed to avoid just this sort of issue. Therefore you can't share their email address with him before they book.
The host doesn't need the email address he just needs to have a setting that all guests are fully verified through photo ID and social media.
Why would he need to get in touch with a guest directly if you are managing the property for him?
And if he is set up as a host on BnB I don't understand why guests wouldn't be able to pay him through their website.
Either the host hasn't fully read through BnB to understand how it works, or more likely this is a scam.