Plant will die. Lophophora is extreme susceptible to infections. For the
best ( if you have spot of decaying plant near the root) you need to
remove it from soil, rinse well with warm water, dry up for 2 days in
Then make a "surgical" removing of decaying part, spread sulfur powder at
the cut. Dry plant for more than a week. Check if operated site is clean and
firm. Plant it in clean soil. Start to water only from bottom of the pot (
drainage hole) and very accurate. Plant usually comes back to life in a
month. This is very serious procedure.