There is a variation on this story involving Schoenberg. Gershwin asked Schoenberg to take him as a student and Schoenberg said something like "I'd only make you a second-rate Schoenberg, but you are a good Gershwin already."
I don't know why Gershwin would have asked Ravel for help, the contrary is more likely. Ravel's final piano concerto was written heavily under the influence of Gershwin. Gershwin pulled a number of people through into jazzy/bluesy American influenced post-classical music, including even Rachmaninoff.
I do not remember who made that film. May be the script writer thought, most of the audience will know who Ravel is but who is Schoenberg?