[Continuing on the theme of getting paid for testimony, we now look at a conflict of interest.] Any testimony that a person would get benefit from, or has some aspect of involvement even if distant, he is disqualified from testifying.

It is written (Yechezqeil 18:18), “he did that which is not good among his people”, and they expound [this verse] (Shevuos 31a), “this is someone accepts authorization” to plead for another. This is specifically when the two litigants are in the city but, for example, the borrower is powerful and a person [who could create] difficulties and the borrower is afraid to complain against him and authorizes to another — [in that case, the person who accepts authorization is] someone who enters a fight which isn’t his. However, if the defendant is in another city, and the claimant is unable to bring himself and [therefore] authorizes another — this one who accepts authorization is doing a mitzvah, for he is returning a taken item from the person who took it. And some say, someone who comes with authorization in order to gain from the reward, and not because of [the other’s] prominence, it is permitted.

Being someone else’s representation in court is permitted, but in general is looked upon unfavorably. Who wants to harm the other litigant?

That said, the Arukh haShulchan lists these cases in which harsha’ah, authorizing legal representation is commended:

1. where the parties are so angry each other that they can not speak productively in each other’s presence (Ritva). The AhS adds that some only recommend representation for the defendant in this case.
2. where one party lives far away and can not get  to the court (Raavad). We saw this case in the Qitzur.
3. where the representative doesn’t find entertainment in being unyielding (Tosafos, as understood by the Rama).
4. where the defendant is not known as being particularly unyielding. Because if someone goes up against an unyielding litigant, the representative is bound to get personally involved in the dispute (Tosados, as understood by the Shakh).

The Arukh haShulchan holds: 5. All of the above.

The Qitzur also appears to raise the Shakh’s case of the unyielding or difficult litigant who is bound to get into a major fight and excludes it as laudable grounds for agreeing to be representation. Although he does raise a “some say”, and excepts the case where it is a professional lawyer just accepting the job for a commission. Presumably because that eliminates the person who is doing it just because they enjoy a fight getting sucked in.