Anyone who lives in a country where security needs compromise human lives (like the case of Israel, if Israel were defeated, Chas veShalom, Israelis would be murdered) has a moral duty to serve in the army. Nobody has the right to stay safe at home while others risk their lives to keep his life safe.
So anyone who discourages a friend from serving in the army is a very bad counsellor, not really a friend.
But if someone refuses to serve lest he is forced to act against his own brethren, as it happened in Gush Katif, then he is justified. But I think that even in such a case, one should serve, and refuse orders when told to act aganst his brethren, even if it means prison.
(Please note that I am speakung specifically about Israel, or some other country which is in real danger. It has no sense to serve in the army of a country which just makes war on a basis of "national pride", like the Argentinian dictarorship which invaded the Falklands)