這句話lz寫得不是很準，應該寫作
No Better Friend , No Worse Enemy.
相當於說，
You cannot find a better friend , if a marine sodlier is your friend ;You cannot find a worse enemy, if a marine sodlier is your enemy.
如果把海軍陸戰隊士兵當作朋友,他就是最好的朋友;如果把海軍陸戰隊士兵當作敵人,他就是最壞的敵人.
也就是說，壹個人的好壞取決於妳對待他的立場，而不是他本人。對應的成語倒還沒想到。
請參考google的如下相關資料
No Better Friend, No Worse Enemy.
Motto of the 1st Marine Division of the United States Marine Corps used by General James Mattis in a message to the troops before the 2003 invasion of Iraq, as quoted in War Stories: Operation Iraqi Freedom (2003) by Oliver North, p. 53; said to be derived from Sulla's famous self-made epitaph.