I was an MP, he would only have jurisdiction on a Military Installation, while on duty. The only exceptions I have run across were at Fort Riley, KS where we would go out and ride along with the Manhattan PD. While out there, we acted with the same authority as the PD. I once had to fly to Benning to pick up an AWOL solider, got to fly armed, good times. Other than that, your buddy is asking to be arrested a could face Civil liabilities.