I would say that corn and rice may be all that he has to offer for the moment.  It's not the worst things, but long term it's not the best either.
As for Garbonzo beans, yes, those can be given too.   I would cook them or if they are canned, rinse well to remove any excess salt.  Smoosh them up a bit.
Changing up feed can upset the digestive system, you are correct, so you may want to provide what the owner gave you and slowly add in some poultry feed if that's what you wish to do.  But egg is easy to give, it's good for them, has protein and after all, baby chicks subsist on egg for the first 3 days of their life. 
Just be sure to cook the rice and beans.  While chickens could eat uncooked rice, they do need a source of grit (Crushed Granite) in order to process something like uncooked rice.  Beans, certain ones can be fed "raw" or "dry" but it's usually better to provide them cooked to avoid any issues.
Hope this helps.