I do not think that the BRICS countries will agree to use the national currency of one of their members as a common currency for mutual settlements. The synthetic currency R5 (real, rouble, rupee, renminbi, rand) has more chances, but this question is still open.
Who will ask them? 
)) They will be so cute to save the Chinese economy with their pockets! China has not just gathered the most ... well, to put it mildly, not smart "friends" ... Just watch!
Do you remember, about six months ago, when the hysteria "everyone is leaving the dollar" and "BRICS are switching to local currencies" began, I gave an example of Brazil-China mutual settlements. And even then he hinted that the Yuan would be the currency of the BRICS. Because even at that moment, the yuan was 80%+ in settlements between these countries. And now the situation is becoming quite clear - ONLY RMB! Just watch these "Chinese Rescuers"

It is in the countries of the Big Seven that no one asks anyone - as the main hegemon said, it will be so. And in the BRICS countries, cooperation is carried out on mutually beneficial terms, no one looks down on anyone, and there is no single decision-making center. In a sense, it is a peer-to-peer hierarchy, like the Bitcoin network. Perhaps that is why so many countries want to join the BRICS that they are tired of external pressure and manipulation attempts.