Realistically everyone trading multiple contracts gets slippage at some point but yes - it is rare for him to get slippage. ES very liquid.
Interesting. I don't know how your friend trades, but I imagine the results would be different for an intraday 'swing trader' and a scalper moving quickly in and out of the market where ticks actually matter.
While generally very liquid - the market can be thin at turning points and certain levels - which is where I've gotten partial fills on my cute 5-lot.