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Generally it takes a lot of valium to cause death by itself (The dosage that will cause a lethal dose in 50% of mice is 720mg/kg). This would translate to over 4,800 tablets to achieve this dosage in a 150 lb human, though the lethal dose may differ in human than rats, so it's hard to say with any degree of accuracy. Obviously, LD-50 studies are not performed on humans.
The problem, though, is that complications from a valium overdose can be fatal or harmful in the least, so he needs emergency treatment. It would be best if you can talk him into going to the ER; if not, it is your obligation as his friend to call 1-800-SUICIDE or 911 to report him. Depending on whether or not alcohol or other medications are involved, this could kill a person.