Cut off all the roots with icky bits and repot in soil that drains very well and fast. If you have the time, water it at least 3 times a day with plain water til it comes out the bottom. The idea here is to get rid of any pathogens remaining in the roots and not have it stay in the soil. You'll know within a week the outcome.
If you start to see new growth at the crown, pot it up in regular soil.
