You're daughter may already have her seat allocated if she's leaving tomorrow.

You could get her some Rescue Remedy from Boots. It's herbal & comes in both pastilles & liquid with a dropper. It's good for anxiety.

Hopefully once she's on the plane, which will be spacious & have seat back TV'S she'll settle down straight away. The flight attendants will be round with drink, snacks & meals so plenty to keep her occupied.

Let us know how she gets on.

Here is a seat map http://thebasource.com/seatmaps/789/8f42j39w127y.html The plane for BA109 is a 787-9 , the seating in economy is 3-3-3 except the last row , you can select your seat free of charge 24 hours before departure , just go into manage my booking , you can change the pre selected seat you have been allocated depending on what's left !!

I flew economy with BA to Dubai in October. We did get the last row of 2.Its unlikely to still be there as a choice now though. I think she would be better now getting seats nearer to the front. Take a bottle of water into the plane. BA is now getting very into cost cutting and may just supply a little cuplet of water. No snacks now apart from the second meal which may be a fun sized chocolate.
I agree with Glynis re Rescue Remedy. I have used it myself when I found I had a panicky flight after years of no problems. The following flight was fine after using Rescue Remedy.
I have made up a play list on my ipod of soothing music specifically to use to help sleep on planes. I put the sound right down and it helps relax me, even i I dont sleep.