Your "client" is not being detained and is free to leave at any time.
However, while he's parked here in this deserted commuter parking lot in this high crime area at 2 in the a.m. I am going to take the opportunity to explore this "reasonably suspicious" behavior by asking your client some very basic questions like "Who are you" and "What are you doing here"?
Your "client" may answer those questions here or at the police station. What do you suggest he do?
I suggest you prove the "high crime" arguement other than relying on the word of the fired officer. Why don't you just face the fact the officer was in the wrong.