It could have been a play because it basically showcases the three characters--Hannah, Iris, and Jack---talking. I'm not sure if it was one.
Now that I've done some research, it wasn't a play. A review on IMDB states that there was plenty of "impromptu conversation Director Lynn Shelton nurtured and encouraged from the mere 70 page script. She confessed that 75% of the dialogue was improvised allowing a unique honesty to develop within each character, thanks to a stellar cast."
Emily Blunt (Iris, in the middle of the photo) is really captivating to watch. More so than her other movies of late like "The Five Year Engagement" or "Salmon Fishing in the Yemen."
What happens is absurd / extreme and so it's somewhat of a farce. And I'm not going to give it away but will ask one question:
What Hannah does is so unethical, how could Iris or Jack forgive her? In real life, I think these incidents would cause them to scatter to the farthest ends of the earth. Instead in the last scene, they are holding hands in a bathroom.