Buffy: I went to Angel's last night and Faith was there. They looked sort of intimate.
Willow: No way. I know what you're thinking and no way!
Buffy: You're right. Faith would never do that.
Faith would never do that? Willow points out - accurately - that Faith would very much do that - that it's Angel who wouldn't.
Yet Buffy thinks that Faith - who by this point has framed Buffy for Finch's death and assaulted Xander seriously enough to leave bruises on his neck - wouldn't do that. And she thinks Angel might.
Buffy being concerned about Angel's faithfulness is hardly something new - see her reaction to spotting Angel and Drusilla together in Lie To Me. For all her romanticization of the relationship, she never really trusts Angel. Angel's return to evil after losing his soul could only make her mistrust worse. Buffy also has a tendency to channel her overall mistrust of Angel into anxiety that he is cheating on her, rather than his pattern of stalking and history of murder. In fact, Angel is romantically faithful to Buffy. But in a myriad of other ways, he is not a person she can trust.
It's the trust she expresses in Faith that is more startling. Surely at this point Buffy has reason to believe that Faith is capable of any manner of betrayal. And yet she doesn't. There is something to that slayer bond - it's one of those relationships that no matter how terrible it gets, even through attempted murder, there is a connection and understanding there. Even at the end of Consequences, Faith saves Buffy's life. Even after Buffy stabs Faith and puts her in a coma, they have a shared dream with affection and trust, and Buffy's first action after awakening is to kiss Faith on the forehead.
In Enemies, Buffy's trust in Faith is very much misplaced. But it speaks to the continued connection - and identification - between them.