Dark Avenger has more attack skills and a summonable kitty, so it's better for PvP. (For a tank)
Paladin has some pve, small party/solo friendly skills, and fits in the middle of tank offense and defense.
Temple Knight just has more personal Defensive skills and highest shield blocking rate. So as a 'pure tank' for Raids and big groups w/healer and such the TK works well. (You'll probably want that Health SA weapon though.)
If your friend is not going to buy an endgame toon (which is illegal and potentially bannable), it will take a huge amount of time to level a tank. Noone (not even NCsoft developers) can say now which tanks whould rule then, and why.