In fact, if I didn't have any further info this smacks of a company in serious financial trouble. It is the type of thing a business might try and do to have cashflow for a very short period to pay some pretty urgent dues. Make sure she refuses to accept the letter.
The plot thickens. The letter that has been sent to all members of staff has absolutely NO details of any pay being withheld. It's just bullshit company propaganda saying that in their staff survey, "one of the biggest requests was for a new payroll system - well we listened."
Also offers details of bridging loans for those who will experience financial hardship.
Turns out some of her colleagues have been talking over the weekend and some of them are prepared for war over this. I'd love to be a fly on the wall at her workplace tomorrow, especially as there's about 10 chefs, most of whom don't mince their words.
Edit: She's now spoken to a boss this morning, who, using his best company language, explained they're not losing the 4 days pay, what's happened is their pay date has been moved along by 4 days, so in affect there's always 4 days in lieu... Ah, ok. That's fine then, thanks for clearing that up.