That is interesting, I have never been in this situation because it seems awkward to bring it up explicitly. That is why I think it is better to simple expect people to pay for what they consume, so it may be implicitly understood that they may be budget constrained without it being a topic.

I imagine most people who are budget constrained simply opt to decline an invitation to meet with friends if they know the average bill will include an extra couple drinks they cannot afford. To me, this is a worse outcome than spending the very little mental effort required to allocate expenses individually.