1) A friend who encourages extreme behaviour to 1) make themselves feel virtuous in comparison 2) make their companion feel as though everything they are doing is really not so bad after all. Both friends have to collude in the behaviour partnership for the relationship to work.
A shopaholic and an enabling friend will go shopping together. The enabling friend (EF) will buy two items and tell the shopaholic things like the real reason they buy is that they have excellent taste, love beautiful things and really deserve them because they work so hard and encourage the partner to buy ten things. The EF then reassures herself or himself that (s)he's very restrained and frugal compared to his or her companion.
The same applies to going drinking with someone who has an alcohol problem.