What I mean by this is that does the effect in battle stack? (for example if there are three FG Pokemon on one side in a triple battle, does everyone take 50% damage?)
Friend Guard does indeed stack in Triple Battles. A friend of mine helped me test this.
First Battle (one Friend Guard): My Talonflame survived a STAB Brave Bird from the other Talonflame. She lost 95 HP out of 162.
Second Battle (two Friend Guards): Same procedure. However, my Talonflame lost 78 HP out of 162.
While they don't stack exactly, seeing as attacks never do the same amount of damage consistently, the effects of Friend Guard DO stack in Triple Battles.