He said it was a FOWLR. Yeah I care for the fish, he does all the maintenance.
Look into the various types of triggerfish (they're territorial but you should have plenty of rock work and caves) and lionfish. In general, they're hardy and will also be large enough that your grouper can't eat them. Lionfish sometimes need to be trained in order to accept frozen food--you may have to start a new one on ghost shrimp and/or damsels (never goldfish).
You actually have a lot of options. Basically, you want fish that meet the following requirements:
-Hardy and known to do well in captivity
-Too large/venomous to be swallowed by the grouper
-Doesn't require coral to be "happy"
-Isn't a slow feeder (the grouper will end up taking all the food)
-Doesn't need to graze on algae, Tangs being an example (most FO or FOWLR systems don't have enough, especially new ones)
-Tropical (should be obvious but keep in mind that coldwater fish do make it into the pet trade sometimes)
Just remember that triggerfish bite and lionfish sting--handle with care!