40 years of bar business experience is not enough? I don't know Franks logic, I can only speak for myself but I think (especially based on this thread) that maybe the USD thing is part of a "weeding out" process. Meaning, its a small place and who the hell wants it's seats taken up by cheapo, complainers who will be counting every basis point on the currency exchange. That type of customers takes up a seat for hours sipping on warm beer. There are already a gauntlet of those bars selling drinks at a f#$%-all profit. You can save even more pesos on beer just sitting outside the bodega near Crazy Ranch. I say, smart move!
I think this is a very good point, and probably the reason for USD menu (and a bit higher pricing). That should sure work, well spotted. I hope he has enough friends and customers of the other kind to keep him afloat - only time will tell...
But the "counting of the exchange basis points" you mention is in fact done by the owner of this bar, imagine you go for a burger today and it is $10 * 49.95 = 499.5 pesos on the bill, then go tomorrow and it is, say, $10 * 50.15 = 511.5, although 12 pesos is 12 pesos (thank you very much dear customer), but I hope that is all that keeps his margins and business going (if that is what he thinks it takes to succeed here and what he is worried about !).
No point really discussing this further until next year, if he renews his lease, then he has made a success of it, if not, well another one bite the dust as they say here in soswer....