How about setting up an email filter, maybe one set to mark as spam or delete if it includes his name in the body (if you don't receive other emails you want with his name in them).

Does she know they're from him before she opens them? I assume she doesn't, since she'd probably delete them if so. If she has any suspicions, she could try scrolling to the bottom, in case he signed it.

She could also get a new email address, and then an alias, and if he finds out her alias, she could easily change it without having to change her real email address.