If you're running IPCop on the router, you should be able to run tcpdump on it and check the firewall to see if it's getting in. If she's got anything more than a dumb router, you may also be able to check traffic there.
My first advice would be to verify your traffic is making it to her router, then see exactly what it is on her router that's breaking it, then make sure it crosses the router to her computer okay.
I don't imagine that any ISP would be dropping TCP/3389 crossing their net, so that's probably out as a cause. I'd check to make sure you're natting (external):3389 to (internal)
3389) on her side properly, and if you are, then start making sure that RDP is set up correctly on her machine.
Out of curiosity, would you be able to take your machine to her place and test connecting local-local without her router in the way, just to verify it works properly on her end?