I'd be very careful of providing or accepting any advice suggesting it might be OK (for visitors) to accept compensation of any kind for "working" in the U.S. All it would take would be an innocent reporting to the IRS, a flag goes up with the INS/USCIS, and the Brits' visitors' vacation could be permanently shortened. FWIW the IRS has a threshold of $600 before reporting is required, but I've been asked for a W-9 form for as little as $50. (A W-9 gives a payor the info they need for IRs reporting &/or backup witholding).
Separately, for many years the IRS has had a (self) reporting requirement for long-term visitors before they leave the U.S., although I've never seen or heard of it being enforced. Search this board for prior discussion of "sailing permit".
Bottom line, it's not worth risking the possibility of running afoul of the U.S. government in any of its forms.
Edit: Clarifications in (parentheses).