Conversation so far:
Whose mistress was Lillie Langtry?
The Prince of Wales became infatuated with Langtry, and she soon became his mistress
How did Lillie Langtry and The Prince of Wales meet?
The Prince of Wales, Albert Edward (Bertie, later Edward VII), arranged to sit next to Langtry at a dinner party given by Sir Allen Young on May 24, 1877
How long did Lillie Langtry and the Prince of Wales' affair last?
Lillie's liaison with the Prince lasted from late 1877 to June 1880

Question: Why did their affair end?
Self-contained rewrite: Why did Lillie Langtry and the Prince of Wales' affair end?