I'm not sure there is anything available like that. If you had access to a mail server it would be fairly trivial to write a script to do just that. Beyond that, it gets a bit tricky.
This link has some info that might help, although it does require an app installed.
If she has thunderbird as an email client there are plugins that will automagically encrypt things
http://en.wikipedia.org/wiki/Enigmail
Or if she has a Mac
http://en.wikipedia.org/wiki/GPGMail
Setup an alternate gmail account that autoforwards to your real gmail account and you are done. However it will probably keep the original header.
Another solution might be to set up a message board online that only you two have access to that sends notifications when you get a message. I'd look at Proboards
http://www.proboards.com. Set it up with an innocuous name like "Mechwarrior 3 fanboard" or something along those lines. Then when she posts you a message on there, it will send you an email saying you have received a message. You then login to the board and check the message there. To most people it looks like someone responded to a posting.
Or, you could get a free webhost and set up a web form that she can use that will automatically encrypt the message and send it to you. (Just need 1 field for the message, make a script that encrypts it however you like and sends it to your inbox.)