A friend of mine would like to travel outside of Poland for the December break,can he travel only with his decision letter, if he have not gotten my TRC?

Decission letter legalize the stay in Poland but it not a document authorizing to cross the border. With the decision, you can legally leave Poland straight to your country of permanent residence, there will be no possibility of transit, e.g. from Poland via Germany to your country of permanent residence. A residence card with a passport entitles you to cross borders.