Final answer:
The next nursing action is to stop the infusion.
Step-by-step explanation:
The next nursing action when the laboratory reports a magnesium level of 6.4 mEq/L (0.30 mmol/L) for a client with preeclampsia who is receiving magnesium sulfate 2 g/hr is to stop the infusion. High magnesium levels can lead to magnesium toxicity, which can cause serious side effects such as depressed reflexes, decreased level of consciousness, and respiratory depression. Therefore, it is important to stop the infusion to prevent further complications.