I see your point, but I wonder if it is not more of a case of the straw that broke the camel's back - a build up of disappointments and broken promises from Frank. Often in relationships, little things become a catalyst and cause a blow-up which may seem out of proportion to the passerby, but really indicate a subterranean pressure in the friendship that has built over a long time before the eruption? And Mary is always trying to paper over the cracks, because she likens their togetherness to the ideal world she envisioned way back on the rooftop - and of course, Charley , being more naturally compassionate than Frank, withdraws his anger too often, for her sake. The complications of long-term relationship festering?
So I wonder if the issue is not the movie deal itself, but that as a sign to Charley of a whole pattern he has resented for years?
One other interesting thing, for me, is that, in youth, Frank has the magnetism of an idealist, but not the integrity of the authentic idealist. Charley and Mary lack the surface charm, but each in their own way have more authenticity about what they hold to be important. Charley's anger helps him survive and remain true to his beliefs; Mary can't be angry without condemning her own illusion of Frank, and so sinks into alcoholism.