How is he getting turn 2 charges off to begin with? even being beasts, if you deploy slightly backwards and shuffle away a bit, turn 2 charges should be an uncertain event.
HYMP broadsides shred spawn rather easy, you just need to defend them from rapiers.
Fire warriors cause Spawn to fall rather fast as well, but generally dislike wyverns shooting at them.
A riptide wing of dakkatides (HBC/SMS/stim/whatever) will probably be the bane of heretic armies, they do good job cleaning them up, and nothing heretics has is really good against them. even the rapiers wont do too much, as it just takes too many HS slots to field them in noteworthy numbers. (a maxed CAD can only get 9 guns, with militia training thats 6.75 hits per turn, causing 5.625 wounds with 3.75 getting passed invul and 2.5 past FnP, at 210 points if he bought no extra crew-and doing that means the guns are easy kills.237 with maxed crewed, and maxed crewed means that after the third wound for a squad, you are crewing at guns.)
Ghosts also make short work of anything renegade, the OSC will do wonders as its not only the (not amazing) stealths, but the ghost as well that puts out some very impressive firepower. (the ion raker alone deals 3.3 wounds to the spawn), and will be good still against everything else in his army too.
Network drones laugh at your burst cannons with their increased accuracy (14 point drone scores 0.75 wounds on the spawn, so 4 drones for 56 usually kill a spawn in a single volly)
The question is not only what he fields-but what you field as well.
Also, is he fielding a legal list?
The IA13 army list has a hard limit on spawn count based on his arch-demagoge.
A mutant overlord unlocks one team, a Tzeentch covenant unlocks 3 teams, so 4 teams is the absolute maximum, assuming he has a Tzeentch mutant overlord. (only works on his warlord! if the arch-demagoge isn't the warlord he unlocks noting!)
there is also a hard limit on Blight Drones, that require your warlord to be with a covenant of Nurgle, and then yo can still take a single squad.
Only way to have both covenants is to be an Arch-heretic Revolutionary (and even that's questionable and requires FAQ if it makes it free, or grants a second free one unrelated to the first)