There is an LHR flight a little earlier but it's full, that may have changed by the time she lands so maybe worth asking, they should be willing to give a voucher for the coach
The 6.15pm to Glasgow from Heathrow has a couple of seats but the onward journey would be at her cost if that's an option
Otherwise I agree - lounge will count as duty of care but the EU payment will be based on the total delay