Okay so Les Mis the musical was created in 1980.
Lily and James died in 1981.
I feel like Lily really loved musicals and went to see Les Mis about 5 times when it first came out in London. One was with James, one was with James and Sirius and Remus, one was with Remus, one was with Molly, Alice and Marlene and one was with her parents.
Now Lily loved all the songs, but her favourite was Do You Hear The People Sing because it symbolised everything she stood for with the order of the Phoenix.
Remus shared her love of musicals and watched many more after her death, sometimes he would consider buying two seats just to pretend that she was still there with him, but he never did. He saw Rent, Cats, The Lion King, Miss Saigon, Phantom. He paid for the tickets with money that Lily had left for him in her will, under the condition that he would use it to treat himself once in a while. Deep down she knew that it was unlikely that they would all make it through the first wizarding war, but something told her that Remus would survive.
After Lily's death Remus went to see Les Mis again. It was nice to get away from everything for a few hours. Then Empty Chairs At Empty Tables started, and he just lost it. He couldn't hold the few remaining pieces of his heart together as he whispered the words of the song, not with the person on the stage but to Lily and James and Peter and Sirius. He had lost all of his friends in one night, just as Marius had done. He stayed until the very end and at the stage door he met the person who played Marius. They saw Remus and smiled reassuringly.
That night eased the aching pain in his heart just a little. It meant that Remus was able to move on and gradually piece his life back together.