Kiss Me First by Lottie Moggach

Twenty-something Leila is socially awkward and friendless.  She is even more alone when her mother dies. After moving to a new flat in another part of London, she gets involved in a website called Red Pill, where the members debate philosophical issues.  The creator of the site, a man named Adrian, approaches Leila, asking her if she would be interested in helping a woman named Tess who wants to commit suicide. Leila's job would be to impersonate Tess online to her family and friends to make them believe that she still is alive, in order to cause them the least amount of pain.  Leila agrees and spends hours getting to know Tess, her background, and the way she behaves, so she can be successful at her task.  Leila and Tess, however, never meet in person.  After Tess leaves on her journey and Leila's job begins, she finds it's hard to separate her life from Tess' and not be curious about how Tess took her own life.  Leila's character is not that sympathetic, but the overall plot really keeps you engaged.