As m3red correctly suggests, Emirates will, in practice, not shy away from J-F op-ups if it suits their requirements on any given flight/day.

EK do tend to protect their F cabins more 'rigorously' than many carriers, but much less so than several others.

I have flown in F cabins with EK as the one & only pax, and have flown in them where every seat is taken. The former is very rare, and something in between those two scenarios is of course the most common.

Not sure what you mean by taking "full advantage of the service" to be honest I find, IME, that food & drink offerings are the same regardless of loads, as is the F class service & overall comfort. I have, on rare occasions, been bothered a little by unusually-noisy other F pax. But - that apart - I guess the only difference is that crew have more time to chat to pax (and amongst themselves of course !) in a 'lighter' cabin. However this is irrelevant for many fellow regulars I know, who tend instead to be absorbed by the ICE - or simply sleep.

I'm sure you will have a great flight, but my own personal tip for your 'first-timer' buddy would be to not set expectations too high. EK are a good carrier, no doubt. But this is not a private jet or Claridges Hotel, remember