They'll allow him to make the avios booking but they wouldn't let him hold two confirmed reservations for very long.

As for increasing the likelihood - it goes from being something that they may very well do in the interests of looking after a high value customer, to a straight up contractual obligation with EU261 protection (although I believe that the latest guidance is that it only goes as far as the LIN-LHR leg plus duty of care, rather than the full €600).

I'm not sure to what extent they'd put him on the later flights to LAS or LAX but at the very least you wouldn't be paying for a new ticket to SFO. I *expect* that they would (it's cheaper for them than duty of care the next day, especially if they don't rebook on AA for the final leg) but I've no direct experience.

You will be spending somewhere between 4,000 to 7,500 avios more, but you'll be paying less in air passenger duty, too.