The fix is two fold, for each of you.
First, configure AIM to send/receive through a specific port. I don't know where it is in the options, but look around, I know it's there
Second, configure your router to forward traffic on that port to your machine.
For example, your IP address is 192.168.1.10, and you chose port 1234 in the AIM options. In the router configuration, you would then choose the port range of 1234-1234, click the check marks for both UDP and TCP, and enter your IP address it the last box. It will have the first 3 already entered (192.168.1) you just add 10 in the last box. This is how my linksys router works, yours may be different.
Good luck... hope this helped.