Say no and give an excuse like “my coach would kill me if he found out”
The main symptom of dehydration is dizziness/delirium.
Have him drink a glass of water and call poison control. do not induce vomiting if not advised.
Some questions Jim should ask to protect his safety:
Who else is coming?
Will there be parental supervision? (If he's under 18)
Will there be any alcohol or drugs?
Where are they meeting?