I think the whole 1 hour per unit is based on similar “science” as the recommended weekly amounts. It just isn’t a one-size-fits-all measurement. How do I know? Back when I was young and foolish, I blew an average 105ug in Huddersfield nick. It was 2:00am, and I was told to expect a long period of boredom as I was to be kept in the cells until I was below the legal drive limit of 35ug. I blew an average 32ug on the same instrument at 10:30am the same morning. Police were somewhat baffled, but the machine doesn’t lie. (Before the pitchforks come out there were some… extenuating circumstances for driving whilst that pissed, as attested to by Huddersfield Magistrates Court endorsing my license with 0 points. It’s a long and boring story.)
The point is, OP’s friend may well have been fine driving after 4 pints over a number of hours. She may also have been a fuel-injected killing machine.
Either way, as has been said above the only truly safe level is 0 and I wouldn’t flinch in the slightest if this became law.