Asking after hearing this from a friend - apologies for the vagueness:

Originally booked on United from USA to Heathrow
United flight delayed then cancelled so rebooked onto BA direct to Heathrow. Given boarding cards and new e-ticket numbers.
Denied boarding at the gate due to insufficient catering on board - BA gate agent said United shouldn't have moved pax onto BA at the last minute without checking!
United then rebooked them onto a later BA flight which got them into London Heathrow about 8 hours after the original United arrival, and about 5 hours after the first BA flight they were moved onto.

They are claiming compensation off United (not via EU261 as that doesn’t apply, just via their own customer relations policy) as they are the ones at fault, but I just wondered purely out of my own curiosity what EU261 says about the move from United to the first BA to the second BA flight - would this be considered IDB from the first BA flight or would they argue this isn’t BA’s issue since United (a non EU carrier) was to blame initially and hence EU261 doesn’t apply here?