Yes, they were best friends since college, but since she lost her parents, she had become unstable, sometimes mixing fantasy with reality.
In this case, maybe fantasy became reality, and she went over the edge?!

Backstory…

Ashleigh grew up in Richmond, which attracts many young families being on the District line for an easy commute downtown and convenient for weekend getaways. Highly prized for its safe neighborhoods, quality of schools and beautifully manicured public gardens, Ashleigh’s parents, Simon and Lilly, settled there soon after marrying in 1994.

Simon was a lawyer and Lilly managed a gallery. Both were tragically murdered when Ashleigh was 16, in an apparent case of mistaken identity after coming out of the historic Global Community Theatre. At the time, Ashleigh was an honor student, all-county track athlete and voted most likely to succeed at the Waldegrave School for girls.

After a year struggling to cope with the death of her parents and living with family friends, she entered King’s College, thinking it would help her refocus. Meghan was her dormmate and best friend. They created a cipher to make communications sort of a game and more exciting. Often, they would leave crypted messages for each other in shared classrooms, their dorm room or around campus.

No family and no other friends to miss her or even care at all, Ashleigh was Meghan, at least in her mind.

Ashleigh poisoned Meghan with the pie she baked for dinner that night, which was filled with deadly nightshade she gathered in Watford, where it grows wild.

Since they played “dress up” that night, Meghan had left her clothes she had worn earlier. Knowing she had would be the last to see her alive, she put them on to quickly go to the London Eye on a different route then Meghan took home and made sure to be seen as Meghan. She always got mistaken for her, so she figured this would work and confuse the timeline.

If you figured this out, you are ready for a promotion!