How many excess electrons are in a modern SLC flash memory cell?
I found a paper, Direct Measurements of Charge in Floating Gate Transistor Channels of Flash Memories Using Scanning Capacitance Microscopy that says in its introduction:

Indeed, the recent technologies store fewer and fewer electrons, down to fewer
than 1000 electrons for 90nm-node NAND Flash.

This paper is from 2006, so more modern chips may well be down in the range that your colleague mentioned. Both geometry sizes and voltages have shrunk since then.