Given that he all ready was previously a 121 Captain, my bet is that he finishes training, starts getting interview calls from majors within 2-6 months of being on the line, and is gone to greener pastures (e.g. a job at a legacy) within a year of starting back at a regional.

As you said, jB requires the 300-hour lookback, SWA requires 2 years of active flying in the previous 5 years, but pretty much none of the others have codified lookback/recency requirements.

If a "quick upgrade" regional like PSA or Compass is convenient to where you live, that could help you if you don't yet have the golden-1000-hours, as well as soften the financial burden for that time, too.