most require that you apply to be a vendor prior to the beginning of the season (I know that's the case for the Northside Farmers' market and the Westside Community Market, but I suppose others might vary).
Fair Share (formerly MACSAC) might be a resource? They've got some info here for growers: http://www.csacoalition.org/resources/growers/
Do you our your friends know anyone with a business/storefront/etc. in town who might be willing to help out? There a guy who sets up a van in front of Warner Park on N Sherman and sells stuff (I've never stopped, and suspect he's not selling things he grows himself, as he often seems to be advertising things not in season here). Not sure what kind of permit is needed, but maybe setting up a stand in a parking lot could work?
Good luck to your friends. It's always tough to be a farmer, this year is worse than most.