This isn't what "isolated ground" means in the NEC
What your customer has is not an isolated ground -- it's an ungrounded system, which is only permitted for specific industrial applications in the NEC where electrical system availability is critical and ground-fault detection can be provisioned separately. In an ungrounded system, there is no neutral wire, just hot wires and an equipment grounding wire -- this doesn't work well with the split-phase neutral system, so 120/240V split phase services are required to be grounded (neutral on this type of service is grounded at the transformer anyway, for that matter).
An isolated ground system, on the other hand, has a normal neutral/ground bond, but runs a dedicated ground wire to the receptacle ground contact from the main bonding point, perhaps via isolated busbars on its way, but not bonded to the main equipment grounding system; likewise, the receptacle ground contacts on isolated ground receptacles are isolated from the receptacle yoke.
As to fixing this...
Go to your local electrical supply house and ask them for a Square D 4028345850K, as per this FAQ entry, then, with the main breaker turned off, install it into the mating screw hole on the loadcenter's neutral tie bar (the metal busbar behind and below the main lugs or main breaker load connections that connects the two neutral bars together).