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Had a customer with the same problem. If you can you will need to deal with the perimeter drainage with weeping tile into a sump barrell and pump to prevent the ductwork form continually receiving the water and eventually rusting out. Some might suggest placing the sump in the ductwork, but that will only be a temporary solution and the ductwork WILL eventually need replacing once it rots out.
This situation is a serious health hazard to the customer and a potential liability for you as a plumber if you attempt a band-aid fix. The duct work is already compromised and contaminated. The way to remedy this hazard is to convert the system to upflow and pump the underslab duct work full of grout or flow-able fill. If the customer is not willing to fix it properly your best bet is to walk away... The last one I did ran about $12k turnkey but your cost will very. Sorry there is no safe easy fix for this one.