Yes. Claude Monet and Édouard Manet were close friends for nearly two decades, despite the confusion their names still cause. They met in the mid 1860s, painted side by side, and Monet was one of the pallbearers at Manet's funeral in 1883.
Manet was also older, more established, and at first mistaken for Monet's teacher.
Where they met
From the mid 1860s, Manet held court at the Café Guerbois on Avenue de Clichy in Paris, twice a week, with Monet, Renoir, Degas, Sisley, Pissarro, and the writer Émile Zola among the regulars.
The group became known as the Batignolles circle, after the neighborhood.
A summer of painting side by side
In the summer of 1874, Manet visited Monet at Argenteuil and set up his easel in the garden to paint Monet's wife Camille and their son Jean.
Renoir showed up, borrowed paint and canvas from Manet, and painted the same scene from a few feet away. Monet, meanwhile, painted Manet at his own easel.
One difference that mattered
Manet never showed his work in the Impressionist group exhibitions. He kept submitting to the official Salon instead, even as he painted more and more like the younger painters around him.
He supported them privately for years without ever officially joining them.
I think the pallbearer detail says more about the friendship than any of the paintings do.
How it ended
When Manet died in 1883, Monet helped carry his coffin. Renoir and Émile Zola were among the other pallbearers.
See more of Monet's world in Famous Impressionist Paintings.