Definitive Illegal Formation Rules

[Telk84]

I might be thick, but what happens if you play against someone with an illegal formation.
Tonights fixture is for 2nd and 3rd place and I noticed my opponent has set his team in an illegal formation right now.
He's playing:
DL -- DC -- DC -- DC
ML -- MC --MC -- MC
-- -- --- --- AMC
------------ ST ----



Does it end up in him having a 20% possesion in favour? Or does it end up me only having 20% possesion
I'm just wondering if I should make a screenshot on my mobile (since I will be away from my PC tonight) and report him if it turns out to be an advantage for him.

Could anyone give me heads up about this since I dont want to win or lose against someone being an Jack***