Final answer:
The nurse should request the physician to withhold the scheduled dose of nivolumab.
Step-by-step explanation:
In this case, the patient is exhibiting watery diarrhea with significant abdominal cramping, which can be sign of a possible side effect or adverse reaction to the medication nivolumab. These symptoms should not be ignored, especially considering that they have been worsening over the last three days. The nurse should take the action of requesting the physician to withhold the scheduled dose of nivolumab, as this may have caused or worsened the symptoms.