Just to chime in here with my experience: T-mobile allows and has an SMS<->email gateway. No fee that I have run into, other than the standard SMS messaging fees. This may depend on your plan, though.
Address outgoing messages from your phone using standard email convention -
[email protected] . She'll have to figure out how to do that on her phone if it isn't obvious, but with my old Nokia it is as easy as typing on the phone numeric keypad. Just need to find the "@" symbol. Might need to upgrade her phone?
T-mobile SMS gateway is: <ACphonenumber>@tmomail.net
FWIW, I used to have my Nokia 5190 hooked directly to my HS machine, using an older SMS interface. It still works, but in my case it is slow and makes HS 'hiccup' when in use. I suspect it has to to do with the fact that I have 65 configured serial ports on my machine - the code probably cycles through the ports every X seconds and causes HS - and the entire computer - to hang and miss cycles. not good, and I can't get ahold of the author to get the source code for the plugin!
I had set up and used the various plugins here at one time or another, so if you need help, holler. I've got all the files necessary for the Nokia 5100 series GSM phones (and others) to work with it. You'll just need the serial interface cable and phone...
http://board.homeseer.com/archive/index.php/t-91203.html