Hgggghhhh....I really want to think that all of her friends are assholes, but at the same time, the paranoia that pops up after getting betrayed can really mess with how you perceive things. It's not healthy, and the way she's going on about this is wrong. Like, from a probability perspective, one of your friends cheating on your husband is plausible, two makes it kinda odd, and three is pushing it to "maybe you need to reevaluate how you make relationship with people in the first place".
She keeps pulling people in her circle into this, but these are people she is doubting. All it does it compromises her search and makes it more difficult as she's stuck trying to balance her feelings. She need neutral, 3rd party people with zero connection to her relationships to be able to give her information she can actually believe. Everyone else connected, no matter if they are actually innocent and supportive, will not be able to satisfy her doubts.
Get a therapist so she can sort her thoughts out, get an investigator to actually do the digging.