It has become conventional to translate the Pali āvuso with “friend”.
There’s a famous passage in the Mahaparinibbana Sutta where the Buddha, supposedly, establishes a hierarchy in title, insisting that a senior should use āvuso to a junior, while a junior should use āyasmā to a senior. (It’s a bit more complicated than that, but this is the relevant part.) It is from this, I guess, that we use the formal “venerable” for āyasmā and the familiar “friend” for āvuso.
The problem is that āvuso really has nothing to do with the notion of “friend”. In fact it is from the same root as āyasmā, both of them relating to the notion of āyu, age or seniority. And while “friend” and “venerable” have an unmistakable difference in degree of respect, this is not the case with āvuso and āyasmā. In fact there are many passages where they are used interchangeably, or do not denote a strict hierarchy of respect.
As just one example, in AN 5.166, Sāriputta uses āyasmā udāyī, while Udāyī—clearly the junior in both monastic seniority and spiritual attainments—uses āvuso sāriputta. The difference here is not degrees of respect, but mere parts of speech (vocative vs. nominative).
In fact, it seems that the Mahaparinibbana passage is a useful text-critical device, as texts that ignore this hierarchy may well stem from before a time when this rule was made. (However, in my opinion it’s likely this passage is somewhat late, probably from the Second Council, so it doesn’t help as much as it might.)
Anyway, I’m trying out using “reverend” instead of “friend” for āvuso. This has the advantage that it’s similar in meaning to “venerable”, but perhaps a little less respectful; although in English, as in Pali, these things are not clear cut. It won’t work all the time, as āvuso is sometimes used to address laymen, but then so is āyasmā.
It might be a little disappointing to find that mendicants were not, in fact, addressing each other as “friend”, but what can I do? I’m just a translator.