Cesar Millan -- Sued Over Pit Bull Attack ... Dog Didn't Make Grade in School

Cesar Millan is being sued for allowing an unsuspecting dog owner to adopt a vicious pit bull that tore into a guest.

Alison Bitney -- yes it's ironic -- claims she was at a friend's house minding her own business when Gus the pit bull went nuts, biting her arm which caused permanent numbness in her hand.

Bitney, a nurse, says she was horrified to learn the dog had a checkered past. It ripped into its trainer's breast and arm back in 2013 in a 20 minute attack. The dog was impounded, only to be saved when a judge signed off and allowed Cesar to train the pooch.

Cesar trained the dog alright, but Bitney says not nearly enough. She claims the dog graduated prematurely and went to live with a guy who had no experience handling dangerous animals.

Bitney claims to have some firepower behind her argument because she says even the director of Millan's Dog Psychology Center admitted the dog needed another 18 months of rehab.

Jen Woodard, VP of the DPC, tells TMZ ... Cesar wasn't Gus' trainer at the facility -- but she adds, "Gus was removed from the DPC against the strong advice and objection of his trainer, before his rehabilitation was completed." Woodard says that was the owner's decision.