A genuine accusation is not Satanic at all; it is simply (when true) an unremarkable statement like “he has sinned” or “he has committed a crime.”
An accusation the devil is likely to make is different. It has the form “he is untested by life”; “if I were to set the stage right, then he would sin”; “if tempted, he would succumb to temptation”; “he is weak and deserves contempt, and I will prove it.”
The accuser in the book of Job did not make the first kind of accusation: Job was “blameless and upright man… who feared God and avoided evil.” (Job 1:1) But he did make the second, arguing: “But now put forth your hand and touch all that he has, and surely he will curse you to your face.” (Job 1:11)
It need not be a sin to accuse another of a past crime; this is done virtuously every day. But it is a sin to tempt another to commit a future crime, and this is partly why Satan is an enemy of man.
There is a subtlety here that in overcoming temptations, man grows stronger. Is Satan therefore in a way a friend? No, says St. Thomas: “That the demons are useful to us is due not to their intention but to the ordering of Divine providence; hence this leads us to be friends, not with them, but with God, Who turns their perverse intention to our profit.” (ST, II-II, 25, 11, reply 3)