Situation A: You bud C robs some nerdy kids of their lunch money, B sees this, is not victimized but knows that guy is your best bud and robbed his best buds
Situation B: Your bud C borrows your signature baseball bat "to show that nerds some respect" and once again beats up the friends of B, he sees he is using your stuff for it, still e is not victimized
Situation C: Your dearest C does what he always does, and in terror B runs to you and asks why you don't help his friends and keep your hands idle, he can't stop C on his own
Situation D: C beats up B - B comes to complain while you are still friends with the guy who beats up him and his friends
Situation E: C uses your bat to beat up B and you run into the fight, both asking you to help
Of course the situations are more and more escalating(and small scalish compared to politics, but i think simple is already complex enough when it comes to disco). Everytime C is out for trouble against B and his friends you know it, he knows it that you know.
The question is how long will he tolerate your knowing-but-not-acting-yet-supporting. What will watchers from the outside think of your triangle?
Will it be acceptable to them that you say these are friends in different areas of your life, thus you keep them seperated and they should deal with that? Will you be able to tell that reason to B with him "understanding" it?
Generally speaking, whatever makes my enemy stronger is something i don't like- i like my enemies weak or weakening. If a friend of mine supports an enemy of mine to reach his goals that are unrelated to me- doesn't my enemy gain more options and capacities to harm me? Yes or no? Would i like that?
Think on tradeships-mercenaries-pirates on teh somali coasts again afterwards and find your grey area again.