She can take the train from Heathrow to Victoria Station and stay at the Thistle Hotel in Victoria Station. She will be walking distance from Buckingham Palace. She would also be a short cab ride to Westminster and
Big Ben. The Thistle is a very old hotel, but clean and safe. The next morning she can have breakfast at the Hotel and take the train back to Heathrow. We have stayed at the Thistle several times and it is a safe place to stay and in the center of old England. If she has more than one bag I would suggest she leave the other in "Checked baggage" at Heathrow. Just take an overnight bag with her short stay. I am very envious.....Unfortunately overseas travel for us is not going to be possible due to medical issues now. Have a wonderful time!!!