What happens if a person with kidney disease uses epsom salts as a laxative ?

Epsom salts (magnesium sulphate) is not recommended in Kidney disease because of the risk of Magnesium toxicity.
Symptoms of Magnesium toxicity include changes in mental status, nausea, diarrhea, appetite loss, muscle weakness, difficulty breathing, extremely low blood pressure, and irregular heartbeat. Though extremely rare severe magnesium intoxication is manifested by a sharp drop in blood pressure and respiratory paralysis.