They know what she is like and have chosen to stick together for fear they will be next.
Don't get me wrong - I also think they're a bunch of self serving c*nts - bug they don't actually know for absolute certain who's lying.
C is maintaining she saw FB messages from op to her husband. If I'm correct, FB messages can be sent without being friends on FB. She's saying they've been deleted since do she can't produce them. What she's saying about any deletion recovery, who knows.
Op says she didn't send them, but they don't know of any (rational!) reasons why C would like about them. They are likely to be swayed by the shitty but nonetheless believed theory that single women are lonely, pathetic, desperate for attention and a man of their own, potentially predatory etc. So that's their reason behind op sending the messages esp if C says she's been flirting with her dh.
They have to choose who they think is lying - and they've chosen op. They'll dismiss the DH episode as him lying for some reason or another to cover it up. Either to not get shit from C for not telling her about the messages, or feeling sorry for op, or not wanting to cause a big schism in the social group, or maybe contemplating some on the side action at a later poibt, who knows.
The fact that who they've chosen to believe also aligns them with the perceived stronger and higher status person and social contact .... Well, very convenient.