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It can definitely be exhaustion, but exhaustion is not fatal in and by it self.
The history that you provided can be a sign of cerebral hypoperfusion which can be seen with underlying ischemia (lack of blood flow to the heart) or simply a low blood pressure causing decreased perfusion to the brain - this can also be a result of an abnormal heart rhythm (regular pulse can rule against this , but an arrhythmia can have a sudden onset and a sudden termination and can be difficult to catch)
I agree with your assessment for calling 911 - at the very least he needs monitoring and fluids to prevent dehydration and worsening of the symptoms.
Please let me know if you have any further questions.
I wish him a speedy recovery,
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