My daughter is accompanying her grandmother to London Heathrow. Her grandmother has a long wait before her connecting flight, but my daughter is exiting the airport at London. Can she wait with her grandmother at her gate until she departs?
You can only enter the departure area of Terminal 5 if you are booked into a flight departing that terminal on that day. The only way that your daughter can accompany her grandmother is to be booked on another departing flight, even if she doesn't intend to actually fly. For example she could book a domestic flight using Avios and a small fee, accompany her grandmother, and then exit the airport without flying.
Exiting the airport requires either taking the transit train to the C gates and staying on the transit back to arrivals, or requesting assistance to exit saying she no longer wishes to fly.