I'm not exactly sure I understabd your setup
but I'm assuming your friend has the Win98 PC
that is behind a firewall. First, firewalls
can be (and are most often) configured to allow
outbound requests to pass through (probably
use IP Masquerading) so an outbound telent
request should not be a problem.
I also assume you have a Linux box with a dial-up
connection to the Internet therefore is using
DCHP to get an IP address from your ISP each time
you dial in (so you probably get a different IP
address each time you connect). First, I would
NEVER allow telnet inbound from the Internet.
You should look into enabling ssh/sshd on your
Linux box. You friend would need an SSH Windows
client. Look at...
http://www.chiark.greenend.org.uk/~sgtatham/putty/
...for a decent free one. However, your ISP may
not route inbound requests to you unless you are
paying for that type of service (typically comes
with a "static" IP address). So, all of this may
be moot.
Anyway, I know it's not exactly what you are
asking for but I hope this helps a little.