the puzzle piece behind the couch

If this is your situation, instead of letting the weight of the problem fall on you, redirect the situation. Ask why all these people are against friend2. Tell friend1 to explain what's so bad about friend2. There could be something you don't see about friend2 that they do see, or it could be something increadibly stupid that makes no sense (in which case I would point that out to friend1).

You're not a bad friend for not outright sticking up for your friend to another friend, you're just worried about offending your first friend by defending the second.

By asking questions you shift the burden of explaining to the friend who's saying bad things, then you either learn something about friend2 that may make you agree with friend1, or you can properly rebuke what friend1 is saying against friend2.