You need to forward the SAMBA or ftp ports through your router to the NAS drive. So for ftp access, forward port 21, for samba, I think it's 139 and 445. Then you should be able to connect to your external ip.
DO NOT DO THIS!!! Especially with samba. It's an unsecure protocol, and is not designed to be sent across the internet. What sort of system have you got at your friends house? Is it another dreambox? You might be able to use shfs, which uses ssh to securely mount filesystems.
How confident are you with linux?
edit: You could also get this to work just using an ssh tunnel.
Have a look at this page for a step by step guide to getting something working with windows at your mates house.