The words, I told you so, come to mind.
As a comcast customer myself, I've had my own experiences with them and their particular brand of support. But pretty much if the technician's laptop will connect, then you're pretty much on your own.
Likely to be a NIC -> Modem interop issue.
Does she have the same modem as your's at your place? I assume not since it worked at your place and doesn't at her's.
So possibly a generic mainstream NIC 10/100 maybe stay away from the Gigs.