Where is New Testament Bible verse: if you have a friend you will be loyal to him no matter what the cost!
Nothing other than John 15:13 (already quoted) comes to mind from the N.T. (However, from the Proverbs there is 17:17, 18:24, I don't know if that helps any.)
The closest thing I can think of is "Greater love has no one than this, that someone lay down his life for his friends". John 15:13
"A friend loves at all times, and a brother is born for adversity." Proverbs 17:17Loyalty includes many things, some that may be painful. Proverbs 27:5-6