A modern antiphrasis in English is the phrase, "asking for a friend". It's normally used when a person wants to know something but humorously states that they're asking on behalf of someone else.
What would be the simplest way to express this in Latin?
My first thought is simply:
Pro amico rogo.
Would another verb be a better choice, like quaerere?
Your choice of preposition is good ('instead of, on behalf of'), but rogāre fundamentally means 'to ask for something to be done', and hence 'to approach with a request; to solicit, proposition', especially common in the political sphere, e.g. Senātus populum/lēgem rogat means 'the Senate is asking (the people's) assent for the law to be passed'; or as a supplicant, prō vītā rogāre 'to ask for one's life to be spared' - notice how even the juxtaposed prō rēpūblicā has the same meaning. Accordingly, prō aliquō rogāre will be understood as 'to approach with a request on behalf of somebody else' with contextual ellipsis; for instance, here prō quōdam tribūtāriō rogāre refers to a request for a cīvitās 'citizenship'.
As a literal translation, I think the verb we're looking for is scīscitārī 'to request information, to want to know, to be curious', so prō amīcō scīscitor. No other verb seems to be as fitting - even the otherwise similar percontārī is concerned with the semantic role of Patient ('I'm querying you so that my friend doesn't have to').
Perhaps a better option is to flip this around and say amīcus interrogat, quaerit, scīscitātur. It implies and contrasts with the excusing nōn ego; as well, it sounds less cryptic, more direct, more Latin. The literal option above cannot really imply a self-distancing excuse, as the English 'asking for myself' would correspond to the benefactive Dative mihi, and not to prō mē.
But the most idiomatic way to offer an excuse (not necessarily humorous) is using the expression alicuijus causā. It's used in precisely this way in Cicero's De Oratore, where the speaker (a top jurist and Cicero's prof) replies to another's embarrassed suggestion that what they're about to discuss will seem too trivial and obvious to him. Notice how the use of generic volēbam removes the need for a more specific verb:
Ego mehercule, inquit Mūcius, anteā vestrā magis hoc causā volēbam quam meā ('Oh bloody hell, I wanted to hear [= was asking] this before not so much for myself but for the sake of you two, said Mucius').
So in the end I would suggest amīcī enim causā volō (or else scīscitor).
Related Q: What is the difference between the words petere and interrogare?
