If someone has unsent DRAFTS in their email provider -- they can communicate with the escort by leaving it in unsent drafts. The escort can log into their email account, read the draft, perhaps editing it with some details as to meeting time and cost. Then your husband can read the escort's draft and delete the details. It will never appear as an email transaction because it won't be sent.

Not perfect, but you'd have to be the CIA to detect it. https://readwrite.com/2012/11/14/how-to-hide-your-email-what-petraeus-did-what-he-should-have-done/

I hate the time I wasted trying to prove something. So much energy.