Because the re-booking costs are a further $1500 she is unwilling to pay this supplement in addition to her re-booked ticket where she paid $250 extra to travel on a day with Q class availability. As its important she doesn't miss Monday and this is a leisure trip I am thinking to book her a flight on BA with some miles.

Given the peculiarities of the small claims courts in England I feel it makes more sense to go down this route rather than pay another $1500 which one might not get back (Plus a throwaway return is a bit cheaper). The claim will only be half the amount and LH are less likely to defend. In the end, as she needs to fly back on the Saturday as originally booked there is nothing to lose by issuing a claim no matter of the chances of its success because these costs are going to be incurred regardless.