2001 Monte Carlo SS won’t start after battery replacement

[JBP]

My neighbors monte Carlo ss did that for some reason. And he had the dealer replace the BCM more times than I can remember before he ended up selling that thing

 

[ls1mike]

Typical PK3 key. It has a transponder. I am not sure how the system in your buddies Monte works. The only two I know are the resistor key and the PK3 key.
I would try a different key if he has one.