I remember once reading the fine print for Vodem and it came to something ludicrous like $700 per GB over your plan :-?
She could always visit an iCafe, but that would defeat the purpose of being able to check your emails outside
Aside from signing up for contracts on aircards and what not, one of the only things that come to mind is a product like a 3Com 3C886. A router with a built in modem. Not too sure if there's wireless ones.
I've read about converters allowing dial up modems to connect through a serial port, but yet again, not too sure. If so these products are pretty uncommon.
Sadly dial up is being less catered for
Maybe the time for an upgrade to broadband has arrived.
Good luck and keep us posted, might be some cheap way around this you find out that could help some others