We may then lay down this rule of friendship — neither ask nor consent to do what is wrong. For the plea “for friendship’s sake” is a discreditable one, and not to be admitted for a moment. This rule holds good for all wrong-doing, but more especially in such as involves disloyalty to the republic.

[Haec igitur lex in amicitia sanciatur, ut neque rogemus res turpes nec faciamus rogati. Turpis enim excusatio est et minime accipienda cum in ceteris peccatis, tum si quis contra rem publicam se amici causa fecisse fateatur.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Laelius De Amicitia [Laelius on Friendship], ch. 12 / sec. 40 (44 BC) [tr. Shuckburgh (1909)]
    (Source)

Original Latin. Alternate translations:

Let this law therefore be established in friendship, viz., that we should neither ask things that are improper, nor grant them when asked; for it is a disgraceful apology, and by no means to be admitted, as well in the case of other offenses, as when any one avows he has acted against the state for the sake of a friend.
[tr. Edmonds (1871)]

As to friendship, then, let this law be enacted, that we neither ask of a friend what is wrong, nor do what is wrong at a friend’s request. The plea that it was for a friend’s sake is a base apology, -- one that should never be admitted with regard to other forms of guilt, and certainly not as to crimes against the State.
[tr. Peabody (1887)]

Therefore let this law be established in friendship: neither ask dishonourable things, nor do them, if asked. And dishonourable it certainly is, and not to be allowed, for anyone to plead in defence of sins in general and especially of those against the State, that he committed them for the sake of a friend.
[tr. Falconer (1923)]

Therefore, let this law be established for friendship: that we should neither ask for foul things nor fulfill requests for them. For this is a foul excuse and ought not be accepted for any crime, but especially not if someone is shown to have placed themselves against the Republic for the sake of a friend.
[Source]