My friend and I are both connected to the same gateway of our cableinternet
provider (@Home). Direct connection between our pc's is only possible with a
extra route rule.

This was no problem with Windows 2000 (route -p <ip other pc> <gateway>) or
Linux (I thought route add -host <ip other pc> <gateway>.

But with FreeBSD something very strange is happening. He makes the route
(route add -host <other pc> <gateway> <netmask>. After that, he can ping me
ONE TIME, just ONE packet, and after that, the rule has disappeared. Same
for (for example) traceroute: he can trace the route to my host just ONE
time, and voilla; rule is gone again and i'm unreachable for him.

We have tried everything we know. Next option will be to get rid of FreeBSD
and install Slackware, but we won't be very happy with that...
tnx

When asking these questions, it would be more useful to post the exact
commands used and the resulting output.  In this case, that would,
ideally, mean a full transcript (see the script(1) command for an easy
way to produce one) containing the whole sequence (add route command,
show the routing table, ping command, show the routing table again)
and the messages the system printed back.  

As a further note, comp.unix.bsd.freebsd.misc is the newsgroup to use
if possible, as it has tremendously more readers (and posters) than
comp.unix.bsd.freebsd.  The *best* technical support, however is
obtained on the freebsd.org mailing lists -- you can even search their
archives before posting.  Also, you'll be more likely to get a reply
if you don't munge your address.

In this case, I'll take a guess that the "<netmask>" value in the
quoted command was wrong.  In the described configuration, it would be
255.255.255.255 (actually, it could and probably should be left out
entirely), or else the route would interfere with reaching the default
router.  In any case, the real question is why you need a route to
reach each other in the first place; the gateway should certainly be
able to forward between you without the static route...

Good luck.