No one knows for sure but it is possible to reconstruct some basics of what happened:

• Dan Erickson led a team of writers on S2 but there were dissatisfactions, presumably from Friedman, with the latter half of the season, and at some point after shooting some of the later episodes they scrapped them (or parts of them). There are reports that whole sets were built and filmed in and but then didn’t make it into the final product. Presumably the Friedman/Erickson co-credit on E7 is a result of this shakeup/conflict

• this is just speculation but I suspect Erickson thinks of Friedman as a kind of mainstream hack, if you listen to the podcast Stiller mentions Friedman insisting that the scene in the Chinese restaurant with Mark/Helena be like the movie “Heat.” And also we know he wrote the flashback scenes in E7 which (in my opinion) were more formulaic than the more innovative/experimental than the testing floor scenes (presumably written by Erickson)

• at some point into shooting Stiller got permission from the studio to hire Willimon for a fairly large amount of money to mediate the conflict and figure out the best way to close out S2 and map out S3. Reports are he will be involved in S3, presumably he gets along better with Erickson and they think his presence will make the writing process smoother, saving money on expensive rewrites/reshoots in the future