Veskit is 100% correct, both sides can be argued (and have been argued) since the book came out. Here is the summary,
Side 1, Only the front rank gets PF - The BRB says that supporting attacks are always limited to 1 attack, therefore no additional attacks. Sorry, go eat a bloat toad.
Side 2, The LM book says that PF "Grants an additional attack after a 6 is rolled." The LM Book does not limit this special attack in any way and clearly the book is intended to allow PF the extra attacks even for supporting attacks as it is a special attack outside of the normal combat restrictions. You eat the bloat toad buddy, extra wasabi.
So now empirically what are we looking at, a lizardmen horde (10 wide) with sword and shield gets 40 attacks. 20 attacks are the front rank which on average roll 3.3 6's. So with rule 1, the Lizardmen horde gets 43.3 attacks. With rule 2, they 46.6 attacks.
In the game we all know that 1/3 or more of that horde is probably dead before it gets to swing from whatever it is fighting which greatly reduces this number anyway.
Lustria has a huge debate about it if anyone really wants to look into both sides.