peer to peer in Linux

Assuming the shares are already set up you should be able to access the share by mounting it locally. Here is an example:
mkdir /local
mount remote-server:/share /share -t nfs.

/local can be any directory that isn't currently used. remote-server is your friend's computer. /share is the folder being shared on your friend's computer. If you want to make this permanent add this line to /etc/fstab
remote-server:/share /local nfs

If you need to set up the share add this to the /etc/exports file:
/share remote-client-ip(s) (NFS options)
/share is the directory you will share. remote-client-ip(s) are the machine(s) you want to grant access to. (NFS options are any special options). There are several choices. Use man nfs or man share to view the man pages and see the available options.

Once this is done just export the share with the command exportfs.

If you have problems after this you probably don't have the NFS server and NFS client started on your computer. Thereare startup scripts for this. The exact names aren't always the same so just look into your startup folders. You could find out if the right services are avialable by looking for rpcbind, mountd and nfs in your process list. If you are still not successful, make sure that rpcbind, nfs and mount are not commented out in /etc/rpc and that nfs and mount are not commented out in /etc/services. Then if you are on different networks and there is a firewall in between make sure it lets the ports defined in /etc/services for those listings through. It may allow ping and not NFS.

Good luck.