[El moustache]

what would you take to assist a 500pt black templar ally?

[Grindgobble Witherfume]

Depends on what the black templar army is like, and if you want either Daemon Hunter or witchhunter help. I assume that Black Templars are an assult force. You could either take a small amount of grey knights for even more assaulting power, or witch hunters to help with ranged firepower and to hold objectives. Excorsists are one of the best tanks in the game at killing monsters and tanks, so it's always a good idea to have one of them floating around.

[El moustache]

sorry yes i should have specified.

he takes a CC heavy force with a dreadnaught

and i have a small collection of Sisters of Battle i can use and a small inquisitor and retinue.
i have...
1 paladin/cannonness
3 arco-flagellants
20 battle sisters
2 rhino transports
10 seraphim
1 immolator
5 retributors (4 heavy bolters)

i also have a few space wolf units but im not sure if i can use them since their new codex came out...
space wolf units are
5 assault terminators
10grey hunters
10 bloodclaws
1 drop pop
1 lukas the trikster model i use as an wolf guard battle leader

[Grindgobble Witherfume]

In this situation, it's now all based on what you'd rather field. The main thing that would attack me to including the witch hunters, would be the exorcist. You could still take a close-ranged fire support army with the sisters squads in transports and the seraphim's. The space wolves lack transports, so I would say go with the witch hunters. Transports are key, so they'll be better off for you.
Sisters in rhino's with a bunch of flamers and faith can DESTROY all sorts of units with their 'rending' hits. If your cannoness has jump-pack and evisorator, that'd be nice, or just stick the character into one of the rhinos in lue of some sisters (if at all possible). I am unfamiliar with the points costs of the witch hunters list, but really you can throw in what ever you want. But the two sisters squads in Rhinos are key.