Thankfully there are more than two people on earth and this edge case won't happen. Because many people are nearly at zero there are a number of also-poor merchants selling low-end goods, and Bob is able to trade a small amount of labour for the goods he needs, and can work his way up.
> yes, the cost might be some inflation, but predictions of inflation have lately failed
For a time, anything may remain out of sync. But you can't get more money from less workers. You can't redistribute to provide UBI, you have to print it, and that devalues the currency.
Even sillier, most UBI proposals are tied to a consumer index, such that when the currency was devalued you would print and distribute more, but with zero concern for the actual workers whose salaries are not also being inflated to match. Eventually the premium for working will be small enough that you'd be a fool for having a job.
And of course "predictions of inflation have lately failed". Because nobody is giving out UBI on a large enough scale. It's suicide. Even tax-and-spend bread-and-circus providing pols still want to be in power next year, so there's a limit to what they can print, and how widely they can distribute it.