"Quae vero pestis efficacior ad nocendum quam familiaris inimicus?"(consolatio III.V.41)
In truth, what kind of pestilence is more effective at causing harm than a hostile friend?
Ain't that the truth? A friend knows all your secrets.
S.J. Tester translated this as "and what plague is more able to hurt a man than an enemy who was once a friend?"
I do not know where he gets the 'once' from. I think people can be working to undo you while still wearing the face of friendship. Which is the point I think Boethius wanted to make.